Abstract: The error of high-accuracy inertial navigation system(INS) caused by the gravity disturbance can't be neglected.Starting from the physical geodesy and Newton's second law,an error dynamics equation of INS,including the influence of gavity disturbance,is firstly presented in this paper.Then,taking single-axis INS as an example,the position error caused by the deflection of vertical and corresponding characteristics of error propagation in three situation were analyzed.Finally,simulation was done on 1′×1′ deflection of vertical database,and from the simulation result we can see that the horizontal error of INS caused by deflections of the vertical on the sailing course can reach as large as 3 km.