Abstract: Based on polarities of P, SV and SH waves combining with their amplitude ratios, using the Focal Mechanism Determining Method of FOCMEC, taking JiuJiang-RuiChang M_S5.7 earthquake on Nov 26 2005 as an example, the influencing factors such as velocity structure model, focal positioning error, data of the first motion and amplitude ratios, distribution of stations and so on are analyzed.The result shows that fine velocity structure model in the study area is necessary in this method;the velocity error of each layer within 5% is negligible.Within the type 2 accuracy of the epicenter location error and the change of focal depth resulting in little change in incidence angle are of little impact on the focal mechanism solutions.Focal mechanism solutions can also be accurately obtained when the number of polarities and stations is relatively less.Therefore, FOCMEC is a stable and reliable method for determining the focal mechanisms of moderate-small earthquakes.