Systematic calibration technology has the advantages of being independent of turntable accuracy and insensitive to shock absorber deformation, etc. It has been widely used in the self-calibration of middle and high precision strapdown inertial navigation systems. The systematic calibration usually takes the velocity error or the differential of the velocity error in navigation mode as measurement. Combined with Kalman filtering technology, the calculation process of system error parameters can be realized automatically. In the systematic calibration algorithm based on velocity measurement, the estimation accuracy of the bias of gyroscope is limited by the stability of bias of the accelerometer in the calibration process. A variable dimension Kalman filter for angular velocity measurement is presented in this paper. The accurate estimation of the bias of the gyroscope can be achieved only by changing the setting of the filter parameters, and no additional gyroscope bias test is required. At last, multiple repeated calibration experiments have been carried out, and the estimation accuracy of gyro bias in the improved scheme is higher than that in the traditional method. The land vehicular tests also prove the effectiveness of the algorithm.