Strength parameters of rock mass are taken as random variables. Based on the reliability theory, an analytical expression of reliability solution for Drucker-Prager criterion is given. The Monte-Carlo method and FORM (first order reliability method) are used to test the correctness of the solution. Some influences of variation of strength parameters on the judging results of Drucker-Prager criterion are discussed by the use of the solution. Conclusions of discussion show that variations of strength parameters have different influences on the reliability probability of Drucker-Prager criterion. When the coefficients of variation of strength parameters are great, their influences on the reliability probability of Drucker-Prager criterion are obvious and can not be neglected. The conclusions of this paper provide a new way for the reliability judgment of yield on rock mass.