Abstract: In order to determine the slope water content that cannot make slope instability, mainly considering the effect of water content on the strength parameters of slope. The slope rock mass is considered as ideal elastic plastic Drucker-Prager material, the onvergence of calculation is taken as failure criterion of slope to as the basis, the stability factor of slope by strength reduction is taken as measure to the slope stability. Stability factors of slope corresponding to different water contents were calculated using large deformation elastic-plastic finite element method. The results showed that, the plastic zone initiates from the toe of the slope, with the gravity loading, the plastic zone expand from the foot to the top of the slope with the strip shape, eventually forming sliding surface of landslide; water content increase induces the strength parameters of rock and soil slope decrease, thereby reducing the stability of the slope, stability factor of slope is significant in a certain condition of water content; ideal elastic plastic stability FEM combining strength reduction method and convergence criterion can be very good to give water content for certain slope stability factor.