In order to analyze the navigation attitude error of the vehicle inertial navigation system （INS） with a large inclination angle， in this paper， it is proposed that the non-verticality of the turntable shaft is the error source of navigation attitude under a large inclination angle condition. The theoretical formula of navigation attitude error related to the attitude angle of the vehicle INS and the non-verticality of the turntable shaft when vehicle INS with a large inclination angle works is evolved， and the simulation and physical verification are implemented. Study results show that under the condition of a large inclination angle， the azimuth error of the INS with a large inclination angle is the tangent obtained by making the pitch angle multiply the non-verticality of the turntable shaft modulated by the azimuth angle. The roll error is the cosine obtained by making the non-verticality of the turntable shaft modulated by the azimuth angle divided by the pitch angle. The pitch angle error is the modulated non-verticality of the turntable shaft and has no relation with the pitch angle of INS. The navigation azimuth error and roll error increase obviously at a large pitch angle when the turntable shaft is non-vertical. The navigation attitude error of INS presents 360° periodic variation and is independent of the INS roll angle.