In this paper，effect of deflection of vertical on ballistic missile impact accuracy is discussed. By analysing the effect of deflection of vertical in the powered steps of flight the perturbations of the cut-off condition are obtained. On analysing the miss coefficient we arrived at the concIusion that it is necessary to use the cut-off parameters taking in account the earths rotation for computing the miss coefficients. Analytical formulas for obtaining the cut-off parameters taking in account the earth's rotation from those without taking earth's rotation into account are derived. These formulas used to compute the cut-off parameters under any lauch conditions without the integration of the system of differential equations. The cut-off parameters thus obtained can be used to compute the miss coefficients. In comparison with the results obtained by direct integration of trajectory equations the method described in this paper gives a relation error on miss distance of less than 2% and an absolute error of about 30 meters. Finally，this paper puts formulas for computing the miss distance due to def1ection of the vertical and methods for correcting the miss distance of inertial guidance are briefly discussed.