Background: Previous studies demonstrated that acupuncture is a safe treatment in general population, however, the safety of acupuncture during menstruation remains controversial. This study aimed to evaluate the safety of acupuncture during menstruation, using change of menstrual cycle as primary outcome. Methods: women who were going to receive at least 12 times acupuncture in the coming one month regardless of diagnosis, and experienced at least one menstruation during the period were recruited from four hospitals in Northern China from March 2017 to March 2018. Patients were followed up 3 menstrual cycles since acupuncture treatment, and data was collected through a questionnaire at baseline and 3 follow-up visits. Chi-square tests and logistic regression were used to test the associations between timing of acupuncture and change of menstrual cycle. Result: there were 373 female patients included in analysis, of which 208(55.8%) had acupuncture during menstruation(AM group) and 165(44.2%) did not(NM group). Nearly one fifth(68, 18.2%) of all patients experienced a change in menstrual cycle. The percentage of patients with changed menstrual cycle in AM group(19.7%) was higher than that of NM group(16.4%), but the difference was not significant(OR 1.254, 95% CI 0.647 to 2.431, P=0.503) when adjusted for age, history of menstrual cycle and diagnose. Conclusion: This study indicated that the rate of menstrual cycle change did not differ among patients who received acupuncture during menstruation or not. However, the interpretation and conclusion of the results should be cautious due to the limitations of the study. Key Words: acupuncture, timing, menstrual cycle change, adverse events, safety