Erythema nodosum is an inflammatory disease with painful erythema of lower extremities as the main clinical manifestation. It usually occurs on the extended side of the lower leg and often occurs with subcutaneous fat lobular septal disease. It is often accompanied by autoimmune lesions, with a long course and easy recurrence. Its etiology and pathogenesis are complicated. Professor ZHANG Ming-he combined with years of clinical practice experience owns an idea of the main pathogenes is that heat-toxicity entering blood makes liquid into phlegm and blocks blood and skin after having combined exogenous damp-heat and endogenous heat. The main reasons of recurrent and unhealed are excessive blood-heat and damp-heat. The main pathological factors are heat-toxicity. When the disease is diagnosed and treated on the basis of heat toxin, the main line is to clear heat, cool blood and detoxify, soften hard and promote blood circulation and dispel stagnation. Self-made main prescriptions for disease differentiation: Hedyotis Diffusa, Sculellaria Barbata, Forsythia, Peony Bark, Radix Paeoniae Rubra, Radix Rehmanniae Elm, Indian Iphigenia Bulb, Uniflower Swisscentaury Root, Peach Seed, Carthamus Tinctorius, Curcuma Zedoary, Piper Cubeba. At the same time, treat according to the patient's physique and the severity of heat toxin, phlegm and blood stasis. Professor Zhang's experience in treating erythema nodosa has significant clinical effects.