The influence of gravity disturbance vector can't be neglected in high-precision long-time inertial navigation system (INS). In this paper, proceeding from INS arrange equations, we establish an error model of INS with gravity disturbance vector, and discuss on the basis of a case study of single channel how gravity disturbance vector influences INS in three varying situations. The simulation result indicates that the gravity disturbance vector has a conspicuous influence on high-precision long-time INS. On the basis of the comprehensive treatise of the influence of gravity disturbance vector on high-precision long-time INS, combined with the actual situation, we put forward two methods of gravity disturbance compensation.