Abstract: Landslide statistics and research findings have shown that the occurrence time of landslides has the feature of evident period distribution.In addition to rainfall,the moon and sun gravitational tide is recognized as another apparent factor.In this paper,the statistical diagram of landslides in days is gained by analyzing the data of a certain number of slides.Further efforts is made on the deduction of the formulas of tidal stress at surface of the Earth,by modifying a simplified elastic earth model based on the earth tide theory.The mechanism of landslide triggered by tidal stress is discussed in further.Moreover,a rock slope in Nanjing is taken as a example,based on the planar failure surface method,to discuss the correlation between tidal stress and time,and how the safety factor of stability change with time.The results show that,the tidal stress and the factor of slope stability change with time at daily period.Therefore,the characteristic with daily successive variation of landslides and the mechanism triggered by earth tide are elaborated quantitatively.