To analyze and correct the influence of launch point vertical deflection on missile inertial guidance precision, a complete errortransfer model of missile inertia guidance was established. Inertia guidance error characteristic of different range and direction was analyzed by error simulation based on standard trajectory parameter. Projection error of apparent acceleration is the main factor of guidance error, while the calculation error of gravitational acceleration and self-aiming error cannot be neglected either. Based on this error-transfer model, vertical deflection influence can be calculated and corrected, which has contributed to improve missile inertial guidance precision.