Abstract: Improving the precision of fiber optic gyroscope (FOG) in vibration environment is important in the application of FOG. Current research on reducing FOG vibration error are mainly concentrate on improving FOG mechanical structure and reinforcing fiber coil. However, fiber tail length asymmetry also has been demonstrated an important reason which can affect the vibration error of FOG. Firstly, the basic theory of phase shift of FOG caused by elastic-optic effect due to vibration was introduced. Then the length of fiber tail on the left and right asymmetry was shown to be the reason of nonreciprocal phase shift in the condition of vibration. Theoretical formula between the length difference of fiber tail in two directions and the output error of gyroscope was derived. Finally, simulations were performed to show the relationship between fiber tail length asymmetry and FOG output error.