Mr. Li from Zhejiang was diagnosed with advanced lung adenocarcinoma and treated for half a year. After chemotherapy in deep vein of lower limbs, edema of left lower limbs suddenly appeared, and ultrasound suggested deep vein thrombosis of lower limbs, and CT angiography of pulmonary artery (CTPA) suggested embolism of right lower pulmonary artery. How are lung cancer patients so prone to venous thromboembolism (VTE)? Tumor-related VTE refers to the combination of venous thromboembolism in patients with malignant tumors, which is one of the common complications of malignant tumors, including deep vein thrombosis (DVT) and pulmonary thromboembolism (PTE), with an incidence rate of about 4% to 20%, and one of the causes of death of tumor patients. Pulmonary embolism is a general term for a group of diseases or clinical syndromes in which the pathogenesis is caused by the obstruction of pulmonary arteries or their branches by various emboli, of which the most common emboli are thrombi, and the commonly called pulmonary embolism is pulmonary thromboembolism. The risk of VTE in the lung cancer population is 20 times higher than that in the general population, and the risk of pulmonary embolism is 6 times higher than that in the general population. The high incidence of complications of pulmonary embolism occurs within 3 to 6 months after the diagnosis of lung cancer. In terms of tumor type, non-small cell lung cancer among lung cancers has a higher risk of pulmonary embolism, especially adenocarcinoma, which is an independent risk factor for pulmonary embolism in lung cancer patients. In terms of tumor stage, stage IV (NSCLC stage) and extensive stage (SCLC stage) are considered independent risk factors for the development of pulmonary embolism. Deep vein thrombosis (DVT) can occur at multiple sites, including the deep veins of the lower extremities, inferior vena cava, pelvic veins, and supraclavicular veins, and the typical clinical symptoms include pain, edema of the distal lower extremity ipsilateral to the venous thrombosis, and heavy or supraclavicular edema. Serum D-dimer, Doppler ultrasonography, and CT or MRI are helpful in the diagnosis of DVT. Once the diagnosis of DVT is confirmed, risk assessment should be carried out immediately, and anticoagulation should be initiated for patients without contraindications to anticoagulation therapy, and commonly used anticoagulants include low molecular heparin, normal heparin, warfarin, sodium sulfadiazine, and rivaroxaban; thrombolytic therapy should be initiated for DVT accompanied by low blood pressure or hemodynamic instability without a high risk of hemorrhage; and patients with contraindications to anticoagulation and thrombolytic therapy may be considered for catheterization or thrombolytic therapy; and patients with contraindications to anticoagulation and thrombolytic therapy may be considered for catheterization. For patients with contraindications to anticoagulation and thrombolytic therapy, treatments such as catheterization or surgical thrombectomy may be considered. The incidence of pulmonary embolism associated with lung cancer ranges from 1.3% to 23.7%, and the period of 3-6 months after the diagnosis of lung cancer is the high incidence of complicated pulmonary embolism. The mechanism of lung cancer-associated pulmonary embolism is complex. Patient-related factors, tumor-related factors, and treatment-related factors together lead to hypercoagulable state of the organism, which increases the risk of pulmonary embolism in patients with lung cancer. Adenocarcinoma, advanced stage of lung cancer, surgical treatment, chemotherapeutic drugs, and the combination of certain underlying diseases are important high-risk factors. The clinical manifestations of pulmonary embolism are easily masked by lung cancer symptoms, and its diagnosis relies on imaging tests represented by CTPA. Some research results have shown that pulmonary embolism likelihood score combined with D-dimer can effectively identify patients with pulmonary embolism in the general population. Patients with lung cancer combined with pulmonary embolism who do not have bleeding manifestations and are at low risk of bleeding may be considered for anticoagulation with novel oral anticoagulants (NOACs) instead of low molecular heparin, and the duration of anticoagulation should be at least 6 months, and the duration of anticoagulation should be prolonged or even long-term anticoagulation as appropriate. Some lung cancer patients at high risk of pulmonary embolism may benefit from prophylactic anticoagulation, which should be carefully evaluated in conjunction with thrombotic risk assessment models and D-dimer. Mr. Li’s pathologic type of adenocarcinoma, receipt of chemotherapy, and deep vein placement in the lower extremities were all at high risk for venous thromboembolism, and it is not surprising that he developed lower extremity venous thrombosis and pulmonary embolism. Fortunately, after 1 month of low molecular heparin anticoagulation and new oral anticoagulant therapy, the lower extremity venous thrombosis and pulmonary embolism had disappeared, and he is now continuing oncology treatment.