For studying the impact of two main factors, strength parameter c and φ, of soil slope stability on reliability index, three calculation models, i.e., fixed value model, random variables model and random field model, were established, which consider the change of numerical value, the reliability and cross correlation and auto correlation of soil parameters respectively. The Bishop method and Rosenbleuth method are adopted to study the effect of strength parameters on reliability index of soil slope. Several conclusions are drawn as follows: The strength parameter c has little effect on soil slope stability; the strength parameter φ is the most sensitive factor in the calculation of soil slope stability, and its value and variability have the great impact on the results of factor of safety and reliability index. So the provided data of φ should be more precise in calculation. Without considering the cross-correlation and auto correlation of strength parameters c and φ, the result of reliability index is much smaller than the real one in the practical project calculation, and its difference may be served as the safety reserve.