The environment of opencast mine is complicated and many factors influence the risk of the slope. But the connection between the factors and risk is hard to be precisely established. So a new method based on the real-time monitoring which can reflect the safety level of the slope is developed. In this method, a new evaluation system which consists of the specific monitoring items is established. The system refers to alarm value of each item and partitions different intervals to match the corresponding probabilities, and the probability of each monitoring item and the comprehensive probability of the slope can be obtained by combining with weight values and AHP method.