In order to improve the calibration efficiency of vehicle strap-down inertial navigation system (SINS) , a gyro drift on-line self-calibration method is proposed by using Kalman filter. In the condition of the approximate level, this method uses completely observability of north gyro drift to acquire two different positions equivalent north gyro drift estimation value by Kalman filter. Therefore, the true value of gyro drift on-line self-calibration is obtained by two different positions estimation values of north gyro drift and calculated azimuth. A numerical example and its simulation result show that this method is feasible and its calibration precision is acceptable.