Abstract: In view of the uncertainty in basic drilling parameters,through the analysis of mechanical risk mechanism of downhole accidents in drilling,non-probabilistic reliability evaluation method was established for drilling engineering risk based on interval analysis method and reliability theory.This method could calculate risk factor after risk evaluation function and the upper and lower bounds of risk factors were established according to prior information.Risk evaluation functions for several common downhole complexities were set up in the paper while the formulas of risk factors were derived from interval variable algorithm.Taking Well W in some western region as an example,the evaluation results indicated that the method,simple and practical,could satisfy the needs of engineering practice.