Abstract: By superimposing the -and -type general solutions of the equilibrium equation without body force and particular solution of the equilibrium with tidal body force, the tidal stress tensor under free boundary conditions is obtained, and the change of the prineipal stresses and principal stress axes with depth is discussed. In the depth range 0-100km, the deviation of the direction of the principal stress axes in space, may reach a value as high as 10(when =30).The phase distributions of tidal hydrostatic stress, the maximum tidal shear stress and the tidal shear stress along the dislocation vectors of the faults as related to the origin times of the earthquakes have been studied for 70 earthquakes (Ms5.0) occurred in our country since 1957, having correct fault plane solutions.There is no apparent correlation between the phase of tidal, hydrostatic stress and the origin time of the earthquake (pr=0.58), but the maximum tidal shear stress has a certain effect on the triggering of the earthquakes (pr=8.110-2). Tidal shear stress along the dislocation vectors of faults plays an even more prominant part (pr=5.210-2). The phase of the maximum shear stress correlates well with the earthquakes of the oblique-slip type (pr=8.710-4).