Instability of slate slope，that is more common in slope engineering, often accompanied by toppling deformation, occurred frequently due to the unique structure of slate. The instability characteristics of slate slope subject to rainfall infiltration are studied by analyzing the instability phenomenon of the right dam foundation slate slope in Miaowei hydropower station; then the stability theory is used in the instability mechanism of slate slope, and combining the engineering practice with stability theory subsequently. Finally, the key measures for preventing instability phenomenon are proposed and used in the subsequent engineering. The results show that the slate joint characteristics, slope excavation ratio and rainfall infiltration are the key factors influencing the stability of slate slope; reinforcement is needed first to prevent rainfall infiltration and, have a good design of waist reinforcement. The slate slope in Miaowei hydropower station after reinforcement is safe and withstand the impact of flood and blasting, so as to prove that the instability causes of slate slope and reinforcement measure are well analyzed. That are useful for the similar slopes when they are need to reinforcement or deal with the instability.