With the Andsersen medical service model, this paper tries to make a study on the impact of medical insurance decisions on the utilization of medical service. Based on the latest data on utilization of medical service on the part of floating population in the survey year which come from Chinese Household income distribution survey data (CHIP2008), with the annual medical expenditure as a proxy for the use of medical services, the paper makes a study on the decision of medical insurance and its impact on medical expenditures, by using propensity score matching (PSM) to wipe out the effect of selection bias and other mixed deviation. The empirical analysis shows that compared with other groups in our country, the rate of the floating population who has participated in the medical insurance is low, whose participation in the medical insurance will significantly increase their medical expenditure. As far as the effect of the floating populations participation in medical insurance on their medical expenditure is concerned, there exist gender differences. Compared with the female floating population, medical insurance plays a more important role in promoting the medical expenditure of the floating population.