Abstract: Objective To estimate the size of MSM population. Methods Using capture-mark-recapture method to estimate the base of crowd size of target population. To obtain the important parameters of the method through internet survey. To use the method of snowball to recruit investigation objects in the field and carry out a face-to-face questionnaire survey. Results The capture mark recapture result of the estimated base of the MSM population size was86883(95%CI, 36260-137506). Conclusion The method of capture mark recapture based on field and online survey met the basic conditions of their application respectively, the investigation results were highly reliable.