The gyroscope successive start-up error is an important factor leading to the error of low- or medium-precision inertial navigation system. In order to realize the stepwise calibration of the three-axis gyro successive start-up errors, the characteristics of the compass heading alignment error were analyzed. Then, the compass heading alignment error model related to the control parameters, the gyro error, and the heading angle was established. Based on the model, a stepwise calibration method of the three-axis gyro successive start-up errors was proposed. First, a z-axis gyro successive start-up error calibration method with variable control parameters was proposed. And then, the z-axis gyro successive start-up error was decoupled from the alignment error model. It stands to reason that the alignment error model is simplified. Second, a three-position calibration method of x- and y-axes gyro successive start-up errors was proposed based on the simplified model. The heading alignment error model equations under three heading angles were established in this method, and the x- and y-axes gyro successive start-up errors were calibrated. Finally, the validity and performance index of the method were verified by using the heading comparison experiment with the high-precision system, the multiposition experiment with equal angular interval, and the roadster verification experiment, respectively. The method in this article only needs to use a simple rotation device to complete the calibration, and has the advantages of simplicity, autonomy, and feasibility.