Located in Wuxi county, Nanmenwan dangerous rock zone has encountered failure for many times which caused deaths of more than 100 people and restricted local social and economic development. On the basis of previous researches, four sliding dangerous rock masses from the zone were chosen as research objects, and their failure probability in normal and rainstorm conditions were calculated by Monte Carlo simulation. The element at risk of the rock masses as well as their vulnerability were also analyzed based on the actual situation. Finally, according to the computation of the above data, the authors concluded that under normal condition, the casualty risk was 0.30 person, and the economic loss risk was 151,600 yuan；while under rainstorm condition, the casualty risk was 0.65 person, and the economic loss risk was 319,100 yuan. Risk assessment results can be used as reference for disaster prevention and reduction.