The mechanical parameters of rock-soil are random. When evaluating the reliability of the slope, the state function can not be expressed explicitly, thus, the response surface method is applied to solve this problem. A new method of slope reliability analysis is proposed combining the FEM strength reduction method. The results of the strength reduction method are adopted to fit the equation for stability coefficient instead of implicit equation, so a limit state equation is established. The approximate equation of calculation stability coefficient is combined with implicit function and the checking point to decide a new basic point and the sample points. The calculation is performed circularly until the reliability index is converged. The accuracy and rationality of the approximate method, which is different from the traditional interpolation points to generate the sample points for the center of vector projection method, is validated through comparison with the reliability indices calculated by different methods. The analysis frequency is significantly reduced, and the convergence of high degree, and the nonlinear limit function used to analyze the reliable indices is improved. At the same time, the proposed method is applied to the analysis of the stability and reliability of Dashuitian slope on Hangzhou-Lanzhou Highway.