As a common land vehicle motion constraint used in GNSS/INS integration system, the non-holonomic constraint (NHC) can effectively inhibit the navigation divergence of standalone inertial navigation system (INS) especially for micro-electro-mechanical system (MEMS) during GNSS signal outages. The correct utilization of non-holonomic constraint requires a compensation of lever arm between the IMU center and the valid point of this constraint on the vehicle. Non-holonomic constraint is not always valid for anywhere on the vehicle since rotation of the vehicle may produce a corresponding lateral/vertical velocity in motion. However, the lever arm impact to non-holonomic constraint has not been paid attention sufficiently and been researched thoroughly. The purpose of this paper is to investigate the effect of the lever arm error on the auxiliary ability of the non-holonomic constraint. The lateral/vertical velocity and additional position drift error during GNSS outages caused by the lever arm error were studied. Two integrated navigation systems with different grade of IMUs were used to analyze the accuracy requirements of the lever arm. Analysis results show that the forward lever arm is the most important influence factor, and the lever arm error should be controlled within sub-decimeter to ensure the accuracy and reliability of the non-holonomic constraint assistance.