Abstract: Under the action of tidal force of the Sun and Moon a periodic tidal force and the westward tidal drag occur in the Earth,the former belonging to a symmetric cyclic alternating stress results in the tectonic stress which is the order of magnitude of 10⁴Pa roughly equal to static tectonic stress of 10⁵ Pa and plays a prominent role in the triggering of earthquake;the later causes the westward shear stress belonging to pulsating cyclic alternating stress which is the order of magnitude of 10⁵Pa roughly equal to static tectonic stress of 10⁶ Pa and makes an important contribution to the horizontal movement of lithosphere and asthenosphere and is a fundamental dynamical factor resulting in the tectonic movement.