Case: Xiaofeng is a middle school freshman, just 12 years old this year, the head is close to 1 meter 7, smart and studious, love basketball. Before the Spring Festival, Xiaofeng found that the right knee joint around sometimes vague pain, mom looked at the red and not swollen, that may be playing basketball twisted, did not care. A month later, Xiaofeng felt that the pain seemed to gradually aggravate. To the nearby hospital for examination, X-ray film did not find obvious abnormalities, the doctor took into account that Xiaofeng is in a period of rapid growth and development, may be “growing pains”, did not do special treatment, told Xiaofeng to pay attention to appropriate rest. Recently, the pain worsened, especially at night after sleeping, sometimes woke up with pain, redness and swelling on the upper left side of the knee joint, and he went to the hospital at the beginning of April for treatment, and the MRI examination suggested that there was a tumor-like lesion in the middle and lower part of the right femur, and he was diagnosed with osteosarcoma after the puncture and biopsy. Osteosarcoma is a common primary malignant bone tumor in orthopedic clinic, commonly known as “bone cancer”, with high malignant degree, high mortality and disability rate, and mostly occurs in children around 10 years old. Like other malignant tumors, early detection, diagnosis and treatment of malignant bone tumors is the key to successful treatment, especially in the past 20 years, the treatment effect of this kind of disease has been significantly improved by the proposal and promotion of neoadjuvant chemotherapy. However, due to the lack of specific symptoms in the early stage of malignant bone tumors, it is not easy to attract the attention of children and parents at the onset of the disease, and it is often regarded as growing pains or common traumas and misdiagnosed and omitted, which makes many children lose the opportunity of early treatment. Malignant bone tumors are mostly found in long tubular bones such as femur, tibia, humerus, etc., with the distal femur and proximal tibia, i.e. around the knee joint being the most common. The main symptom in the early stage is localized pain, the degree of pain varies, some only have slight soreness or discomfort, the child may not feel pain when playing happily, and feel pain at night when sleeping, like Xiao Feng woke up in his sleep with pain, which is known as “resting pain” in medicine. As the disease progresses, the pain is progressive aggravation, the pain site is fixed, local swelling can gradually appear, the skin temperature is higher than the healthy side, some children may be accompanied by fever, poor appetite, anemia and other symptoms. Generally, children’s pain is not easy to be associated with bone tumor, but pain is a major symptom of bone tumor, especially at night, pain aggravation should be highly vigilant. Therefore, when children have unexplained pain around the knee joint, or the pain can not be relieved after resting for more than 1 week after minor injuries, we should be alert to the possibility of malignant bone tumors, and should take the child to a specialist hospital in a timely manner. Non-bone tumor specialists should be extra cautious when seeing such children. For children with no obvious abnormalities on plain X-rays, they should be advised to follow up or perform MRI examination if necessary, and should not easily think that it is growing pains or soft tissue injuries that are misdiagnosed and omitted.