OBJECTIVE： To study the prevention and treatment effect and its mechanism of Yinyanjing lotion on allergic contact dermatitis （ACD） in rats. METHODS： 60 rats were randomly divided into blank control group， model control group， positive control group （dexamethasone） and Yinyanjing lotion high-dose， medium-dose， low-dose groups （respectively given 2-fold concentrated solution， 1-fold concentrated solution and original solution）， 10 in each group. Except for blank control group， rats in other groups smeared dinitrochlorobenzene in the abdomen and back （2 cm×2 cm） to induce ACD model. From the 9th of modeling， rats in each administration group were smeared 100 μL of related drugs in the back， and rats in blank control group and model control group smeared 100 μL of purified water， 3 times a day， for 10 d. After administration， skin allergic reaction was scored and skin swelling degree was determined. HE staining was used to observe the pathological changes of skin. Enzyme-linked immunosorbent method was used to detect the contents of interleukin-2 （IL-2）， IL-4， IL-6 and interferon γ （IFN-γ） in serum. RESULTS： Compared with blank control group， allergic reaction scores and skin swelling degree in model control group were significantly increased （P＜0.01）. Electron microscopy showed epidermal scab necrosis， dermal layer damage， hair follicle necrosis. Contents of IL-2， IL-4， IFN-γ in serum were significantly increased （P＜0.05 or P＜0.01）； IL-6 content was slightly increased （P＞0.05）. Compared with model control group， except that the skin swelling degree， IL-4 content in serum in Yinyanjing lotion low-dose group and IL-6 content in serum in each administration group were not significantly improved， other indexes were significantly improved （P＜0.05 or P＜0.01）. CONCLUSIONS： Yinyanjing lotion has certain prevention and treatment effects on ACD in rats， the mechanism may be associated with reducing the contents of IL-2， IL-4， IFN-γ in serum.