To address the error problem of the deflection of the vertical(DOV),which restricts the precision of high-precision inertial navigation system(INS),the influence of DOV on the horizontalposition error of INS and the index requirements of DOV for error compensation of INS at all levels are studied. Firstly, the direct difference method and the fourth order Runge Kutta numerical updating algorithm of the INS error term caused by DOV are derived. The updating effects of the horizontal position errors of the two algorithms in different regions are compared and analyzed. Then, the INS is compensated with three kinds of resolution DOV grid data. Finally, the effect of DOV compensation frequency on position error compensation is analyzed and DOV compensation experiment of vehicle navigation is carried out. The simulation and experimental results show thatboth error update algorithms can effectively calculate the horizontal position error. The maximum DOV can cause a position error of nearly 3 000 m,and the horizontal attitude error and azimuth attitude error drift about 18" and 72" in 1 h. After DOV compensation,the horizontal positioning accuracy is improved by about 230 m.