The gyro drift is a crucial factor for initial orientation error and measurement error of SIMS (Strapdown Inertial Measurement System). Accurate ground calibration of gyro drift is an important way to reduce errors of alignment and inertial measurement. Based on the initial alignment principle of SIMS on stationary base and inertial navigation algorithm, the gyro drift models are established. A new method that a gyro drift calibration method based on free double-position ground self-alignment is proposed. The simulation and experiment results show that the method can calibrate and compensate the gyro drift, improve the measurement precision of SIMS, and verify the efficiency of the proposed method.