Erythema nodosum is an inflammatory disease occurring in the subcutaneous fat, the cause of which is complex, mainly caused by infectious factors such as streptococcus, rheumatic immune diseases and tumors. The severity of erythema nodosum is different because of different primary diseases. 1. Infectious factors: erythema nodosum is usually induced by type IV metaplasia caused by Mycobacterium tuberculosis and streptococcal infection, and the condition is usually not serious and will gradually improve through anti-tuberculosis treatment and anti-inflammatory treatment. In addition, diseases caused by Mycobacterium leprae, gonococcus, HIV, etc. can also be accompanied by erythema nodosum, which requires treatment of the primary disease; 2. Rheumatic immune diseases: If rheumatic immune diseases cause erythema nodosum, it is relatively serious, and it is mostly seen in systemic lupus erythematosus, dry syndrome, rheumatoid arthritis, etc. Rheumatic immune diseases will affect patients’ daily life and are more difficult to be cured. Patients are suggested to go to regular hospitals for anti-rheumatic drug treatment in time; 3. Tumor: tumor diseases may also induce erythema nodosum, which is more serious and may affect patients’ quality of life and requires timely diagnosis and anti-cancer treatment. These etiologies are not easy to distinguish only through symptomatic manifestations and need to be clearly diagnosed and differentially diagnosed by doctors based on clinical experience, physical examination, corresponding hematological examination, histopathological examination and other results. Therefore, patients are recommended to go to hospitals to find out the severity of the disease so as not to delay the disease.