Abstract: Objective To observe the clinical effect of the Cai's menstruation periodic treatment in obese patients with polycystic ovary syndrome(PCOS). Methods Total 119 cases of obese patients with PCOS were selected from Shanghai General Hospital and Longhua Hospital Shanghai University of TCM from July 2016 to June 2019. According to non randomized concurrent controlled method, the patients were divided into treatment group(n=59) and control group(n=60). The treatment group was treated with Duonang Decoction after menstruation and in the mid-menstrual period, and before menstruation it was treated with Duo Nang Decoction which was removed spina gleditsiae and increased curculigo, purple quartz and antler frost. The control group was treated with metformin. The two groups were treated for 3 months, and waist-hip ratio(WHR), body mass index(BMI), testosterone(T), follicle stimulating hormone(FSH), luteinizing hormone(LH), LH/FSH ratio, glucose metabolism, TCM syndrome score, basal body temperature(BBT) and related safety indexes before and after treatment were observed. Results After treatment, 22 cases were cured, 4 cases were significantly effective and 25 cases were effective in the treatment group, while 9 cases were cured, 8 cases were significantly effective and 25 cases were effective in the control group. The curative effect of the treatment group was significantly better than that of the control group, and the difference was statistically significant(Z=-2.658, P=0.008). After treatment, the recovery rate of BBT and the pregnancy rate in the treatment group were better than those in the control group(all P<0.05), and the scores of kidney deficiency and phlegm stasis syndrome and LH, LH/FSH ratio in the treatment group were significantly lower than those in the control group(all P<0.05). After treatment, BMI, WHR, fasting insulin and insulin resistance index in the two groups were significantly lower than those before treatment(all P<0.05), but there was no statistical difference between the two groups(all P>0.05). Conclusion The Cai's menstruation periodic treatment can improve the sex hormone level well in obese PCOS patients, effectively regulate the disorder of glucose metabolism and reduce body weight, thus to promote ovulation, recover regular menstruation and prevent forward complications. In addition, the periodic treatment can also significantly improve the symptom of kidney deficiency and blood stasis, and the overall clinical effect is better.