By analyzing the digital tilt tidal observations at Yongsheng station and strain tidal observations at Houma station,this paper suggests that the solid tidal force might trigger tiny abrupt deformation when the seismogenic system enters into an instable critical state. The step and abrupt changes correlate to a certain extent to the direction of tidal force and the orientation and size of tidal shear stress along the fault strike. In the paper, we study for the first time the relationship between deformation anomaly and additional tidal stress in order to approach the physical mechanism of anomalies appeared immediately before earthquakes and the distinguishing method.