Referring to the theoretical analysis method of the interference fiber optic gyroscope (FOG) based on linear polarization, the polarization error resulted from the circular polarization degree, coupling or reflex points, and misalignment of the butt-joint in a circular polarization maintaining optical fiber gyroscope is analyzed. The result shows that the reflex points and the misalignment which easily cause polarization error in a linear polarization system do not make influence in a circular polarization FOG. As the coupling polarization error from coupling points changes exponentially with the loss coefficient, electing a high quality single circular polarization maintaining optical fiber is significant for reducing the polarization error.