Objective：To observe the effect of modified prescription of Xiaoqinglong tang in different doses on nitric oxide(NO)content and nitric oxide synthase(NOS)in asthmatic mice at the attack stage，and to discuss the possible functioning mechanism of the prescription at acute attack of asthma.Methods：Divided BALB/c mice into the blank group，the model group， the low-dose and high-dose modified prescription of Xiaoqinglong tang group and the hormone group randomly，10 mice in each group.Injection with suspension of ovalbumin and aluminum hydroxide gel was administered to establish the model of asthmatic mice.After using the modified prescription of Xiaoqinglong tang in different doses and dexamethasone to intervene the treatment，measured the changes of activity of NO，iNOS and cNOS in serum and lung tissue homogenate of mice in each goup， discussed the relevance between the indexes detected above and asthma in acute attack；and by means of examining pathological examination of lung tissue，observed the infiltration of inflammatory cells of lung tissue in each group.Results：Comparing with the blank group，the content of NO and iNOS in serum and lung tissue homogenate of mice in the model group was significantly increased(P＜0.05)，cNOS was not significantly changed，and the lesions of lung tissue were severe.Comparing with the model group，the content of NO and iNOS of mice in the low-dose and high-dose modified prescription of Xiaoqinglong tang group as well as the hormone group was respectively decreased(P＜0.05)，and the high-dose modified prescription of Xiaoqinglong tang group and the hormone group were decreased the most significantly；the pathological section of lung tissue showed that the inflammatory lesion of the hormone group and the low-dose and high-dose modified prescription of Xiaoqinglong tang group was significantly improved，and the lesion of the hormone group and the high-dose modified prescription of Xiaoqinglong tang group was the slightest.Comparing with the low-dose Chinese medicine group， the content of NO in serum and lung tissue homogenate， the content of TNOS and iNOS in serum and the content of TNOS and iNOS in lung tissue homogenate of mice in the high-dose group were significantly decreased，differences being significant(P＜0.05).Conclusion：The modified prescription of Xiaoqinglong tang can reduce iNOS content and inhibit its activity expression in mice at the attack stage of asthma and decrease the excessive production of NO，thus to improve asthmatic symptoms and the pathological changes of lung tissue，and its efficacy enhances as the dosage increases in certain scope，which shows multiple-approach and multi-target as the clinical advantage of Chinese herbal compound in the treatment of disease.