Factors influencing the stability of soil slopes are analyzed by means of strength reduction method, based on the physical and mechanical parameters of typical strata in Beijing. The sensitivity of the influencing factors are analyzed by means of grey correlation degree theory. The influencing factors studied include the angle and height of the slope, the cohesion, internal friction angle, specific weight, elastic modulus and Poisson ratio of the soil and the load behind the slope. Conclusions drawn are as follows: 1) The strength parameters of the soil mass, i.e., the cohesion and the internal friction angle, have the most decisive effect on the soil slope stability； 2) The geometrical parameters of the slope, i.e., the height and angle of the slope are in the second position; 3) The specific weight of the soil mass and the load behind the slope are in the third position; and 4) The elastic modulus and Poisson ratio have no effect on the stability of the soil slope.