Abstract: The researches on time of landslide occurrence have been the hotspot in the landslide area, and most of them mainly focus on the rainfall season. However, few scientists consider the diurnal period properties of slope stability caused by earth tide. The earth tide is always the hotspot in the geophysics field, and it relates to earthquakes strongly. The formulas for tidal stress at the surface of earth are derived by modifying a simplified elastic earth model based on the earth tide theory. The effect of earth tides on slope stability and its time correlation are discussed by considering the tidal stress in the formulas for safety factor of slope stability commonly used, and typical slopes in Nanjing City are taken as examples. The suggestion of taking the effects of earth tide into account in the protection of slopes is proposed.