In view of the imperfection of the method for determining the threshold value of the dam early warning index and the weak guiding role of the monitoring index, this paper systematically expounds the dam early warning theory based on the Hall three-dimensional structure, and constructs the early warning index system. By transforming the normal distribution calculation of dam monitoring data into the query of Kolmogorov distribution table, a method for determining the early warning threshold of dam multi-source monitoring based on small probability method is proposed.Combined with the observation data of a reservoir, the early-warning threshold of the surface settlement and seepage flow of the dam is proposed. The analysis results show that the method can effectively evaluate the monitoring indicators and the ability to resist adverse working conditions over the years, better reflect the operation of the dam, and greatly improve the reliability of the monitoring and early-warning indicators.