The influence of the source parameters on M_s is studied through numerical simulations based on finite-fault models. The results show that the M_s is negatively correlated with earthquake rupture duration when moment magnitude remains unchanged. The source parameters that can impact the rupture duration contain the sub-fault duration, rupture velocity, and rupture length. Among them, the sub-fault duration and rupture length is negatively correlated with M_s, while the rupture velocity is positively correlated with M_s, if the moment magnitude remains unchanged. The fact that the measured M_s of strong earthquakes in the mainland of China are significantly larger than moment magnitudes may be due to two reasons. First, the strong earthquakes in the mainland of China are mostly intraplate earthquakes with relatively higher stress drop values, which may have short rupture duration and rupture length. Secondly, M_s determined by China Earthquake Networks Center (CENC) is systematically larger than M_s(20) determined by National Earthquake Information Center.