Abstract: According to the uncertainty in the process of calculating the occurrence probability of large earthquakes， a study is made on uncertainty and its dealing methods. Considering the complexity of uncertainty， this paper presents a method for calculating the occurrence probability of large earthquakes based on Monte Carlo simulation. With the Tazang segment of eastern Kunlun fault zone as an example， we deal with different kinds of uncertainties in calculating the occurrence probability of large earthquakes using Monte Carlo method. The result shows that the incompleteness of paleo-earthquakes data has great effect on the calculation result. With the logical tree to deal with the incompleteness of paleo-earthquakes data， the occurrence probability of large earthquakes is 0.12 in the next 100 years on the Tazang segment.