Abstract: In order to solve the landslide risk evaluation of landslide failure instability probability calculation problem, this paper summarizes the research achievements of previous researchers in rainfall threshold and, in combination with the previous research results in a threshold rainfall, uses the theory of statistical to perform coupling analysis of the historical record of the rainfall landslide and rainfall data, so as to create a rainfall landslide failure instability probability calculation model, with Miaoba landslide in Yanjin County of Yunnan Province as an example for demonstration.The results show that the rainfall threshold type of Yanjin County rainfall landslide is cumulative rainfall duration threshold, which means the one-day rainfall threshold, with the rainfall threshold being 29.7 mm; most of rainfall landslide in Yanjin County is caused by the daily rainfall which reaches rainfall threshold within 5 days; the possibility that the rain reaches or exceeds 29.7 mm from August 20th to 25th is 46.49%;the possibility of the failure of Miaoba landslide in August 25th is 0.2853%.