We demonstrate a simulation method to estimate the bias error in fiber optic gyroscope caused by the even order distortion.The bias error can be expressed by the Bessel functions and trigonometric identities.Corresponding to theoretical simulation,the bias error varies dependence on the even order distortion crosstalk and birefringence phase angle.The bias error can be minimized by choosing the modulation depth around 3π./5 The even order distortion is measured by using the spectrum analyzer since the even order distortion is a main factor in the causing bias error.Moreover,the bias error is measured through the output of fiber optic gyroscope.The experiment data of the bias error is fit well with the simulation result.