Abstract: Based on the first motions of P, SV waves and their amplitude ratio, we calculated focal mechanism solutions of 45 times small earthquakes occurred in the Xiaolangdi reservoir area from January 2009 to December 2012.The results show that:45 focal mechanisms contain 13 normal fault types, 15 thrust fault types and 17 strike slip types;the direction of the maximum compressive stress axis(P-axis) is NEESWW (220°~230°), which is not completely consistent with the nearly horizontal tectonic stress field of North China, and indicate the tectonic stress field of North China has less controlling action in the Xiaolangdi Reservoir area.This phenomenon would relate to the complex structure of subsurface media and reservoir water storage.