According to the moving characteristics of tracklayer, when the vehicle is on ramp and the ground's vertical pressure is linear, the calculating formulas of relationship between the deviation of the instantaneous steering center, the position of vehicle's gravity, steering radius, moving velocity, moving acceleration, and vehicle's orientation, were obtained by the theory and method of vector analysis in mathematics and mechanics. Based on the formulas, the cause and rules of the deviation of the instantaneous steering center were analyzed，and the factors of causing instability of steering were indicated.