Abstract： Objective： To observe the clinical effect of Modified Simiao Powder combined with glucocorticoid on polymyositis (PM) and its effect on muscle strength and levels of inflammatory factors. Methods： A total of 60 cases of PM patients were randomly selected as the control group and the observation group，with 30 cases in each group. Both groups were treated with routine western medicine， and the observation group was additionally treated with Modified Simiao Powder. After treatment， the clinical effects and incidence of adverse reactions were compared between the two groups. Before and after treatment， the changes in traditional Chinese medicine (TCM) syndrome scores， electromyogram， muscle strength grading，and levels of inflammatory factors including prostaglandin E2 (PGE2)，leukotriene E4 (LTE4) and prostaglandin D2 (PGD2) were compared between the two groups. Results：The total effective rate was 96.67% in the observation group， and 73.33% in the control group， the difference being significant (P＜0.05). Before treatment，there was no significant difference being found in the comparison of main and secondary symptom scores of TCM syndromes and the total scores between the two groups (P＞0.05). After treatment，main and secondary symptom scores of TCM syndromes and the total scores in the two groups were decreased when compared with those before treatment (P＜0.05)，and the main and secondary symptom scores and total score in the observation group were lower than those in the control group (P＜0.05). Before treatment， there was no significant difference being found in the comparison of duration and amplitude of motor unit potential between the two groups (P＞0.05). After treatment， the duration and amplitude of motor unit potential in the two groups were increased when compared with those before treatment (P＜0.05)，and the duration and amplitude of motor unit potential in the observation group were higher than those in the control group (P＜0.05). Before treatment，there was no significant difference being found in the comparison of ratio of muscle strength reaching level 3 between the two groups (P＞ 0.05). After treatment， the rates of muscle strength reaching level 3 in the two groups were increased when compared with those before treatment (P＜0.05)，and the rate of muscle strength reaching level 3 in the observation group was higher than that in the control group (P＜0.05). Before treatment，there was no significant difference being found in the comparison of levels of PGE2，LTE4，and PGD2 between the two groups (P＞0.05). After treatment，the levels of PGE2，LTE4，and PGD2 in the two groups were decreased when compared with those before treatment (P＜0.05)， and the levels of PGE2， LTE4， and PGD2 in the observation group were lower than those in the control group (P＜0.05). There were no significant adverse reactions in the observation group， and the incidence of adverse reactions was 10.00% in the control group， there being no significant difference (P＞0.05). Conclusion： The combination of Modified Simiao Powder and glucocorticoids has a significant curative effect on PM，which can further reduce inflammatory reactions，help restore muscle strength，and alleviate clinical symptoms.