Abstract: The influence of horizontal disturbing gravity (HDG) on the positional error of high-precision Inertial Navigation Systems (INSs) was studied under different conditions of movement. A space-state equation expressing INS error with HDG was deduced and analytic expressions of INS position error with HDG were deduced under three conditions of movement. Additionally, a method of computing above influence based on inertial navigation calculation is designed. Under the condition of uniform motion, INS position error caused by the same HDG was calculated through analytic expressions and inertial navigation calculations. The results from analytic expressions show that disturbing gravity varied between ±80 mGal (1 mGal = 10^-5 m/s²) causing about 3 000 m INS positional error at the maximum. A comparison of the results from two methods show that the level and change in INS positional error are basically consistent while the validity of each method was verified by these results.