In this paper, the relationship between seismicity in the Northwest China and three components of the Lunar and Solar tide-generating force has been researched deeply.Based on the geological structure in the region, authors let one of two horizontal components of the tidal force parallel to rupture strike and another perpendicular to that.The result shows that there is an important relationship between seismicity and two horizontal components simultaneously,and parallel one is more important than vertical one.This result has more important significance for predicting occurrence time of an earthquake.