The influence of the misalignment angle between the dither axis and sensitive axis of the mechanically dithered ring laser gyro on the gyro accuracy is analyzed theoretically. Based on the Euler dynamics equation of rigid body, the dynamic model of mechanically dithered ring laser gyro is established. The change of the product inertia of the gyro cavity caused by the misalignment angle of dither axis in gyro coordinates is calculated by the calculation method of the moment of inertia of the rigid body about an arbitrary axis. When the gyro dithers at high frequency, the lateral moment generated by the inertial product of the cavity causes the lateral deformation of the gyro sensitive axis, and then triggers the single-gyro-level coning error. This process has been quantitatively calculated in this work. The results show that the misalignment angle of gyro dither axis should be less than 5′ in high-precision applications.