Coughing up blood after chemotherapy for lung cancer may be related to the invasion of blood vessels by cancer cells, adverse reaction after chemotherapy and other reasons, which need to be further differentiated and diagnosed by doctors. 1. Invasion of blood vessels by cancer cells: if the patient’s condition continues to develop after chemotherapy, there may be invasion of blood vessels by tumor cells, leading to the symptom of coughing up blood. 2. Adverse reaction after chemotherapy: lung cancer patients with severe bone marrow suppression after chemotherapy may have low platelets, leading to spontaneous bleeding, which may lead to the symptom of coughing up blood. It is suggested that lung cancer patients who cough up blood after chemotherapy should go to the hospital in time, clarify the cause of disease and carry out standardized treatment under the guidance of doctors, so as to avoid the delay of disease and bring adverse effects to the body.