Objective:To observe the clinical effects of “Qiqing Xiaodian Decoction” on allergic purpura and the level of immunoglobulin and complement. Method 60 patients with allergic purpura were randomly divided into treatment group and control group,30 patients in each group. Patients in treatment group were treated with Qi-qing-xiao-dian decoction, while patients in control group were treated with conventional western medicine(antihistamines and glucocorticoid),with a course of 8 weeks. The clinical efficacy and the change of indicators related to immunity such as immunoglobulin (IgA, IgE, IgM, IgG) and complement (c3, c4) were observed.  Results: ①The total effective rate were 96.7% in the treatment group and 93.3% in the control group respectively. There was no statistical significance between groups (P>0.05). ②The level of IgA,IgE,IgG and IgM within each group before and after treatment significantly decreased (P<0.05)；after treatment, the differences of the level of these indicators between two group were statistical significant(P<0.05). ③Compared within each group before and after treatment, the level of c3 increased significantly in both groups(P<0.05), while the difference of the level of c4 was not statistical significant (P>0.05). After treatment, there were no statistical significance between groups on the level of c3 and c4 (P>0.05).  Conclusion:①When allergic purpura attacks acutely, the level of immunoglobulin (IgA, IgE, IgM, IgG) increase and the level of complement c3 decrease. ②“Qiqing Xiaodian Decoction” showed good clinical effects on allergic purpura with the therapy of supplement Qi and dispeling wind. The mechanism was related to regulating humoral immunity, enhancing the nonspecific immune reaction of the body, alleviating the allergic reaction of vessels and capillary fragility.