As an important sensor in the navigation systems, star sensors and the gyro play important roles in spacecraft attitude determination system. Complex environmental factors are the main sources of error in attitude determination. The error influence of different benchmarks and the disintegration mode between the star sensor and the gyro is analyzed in theory. The integrated design of the star sensor and the gyro on the same benchmark can effectively avoid the error influence and improves the spacecraft attitude determination accuracy. Simulation results indicate that when the stars sensor optical axis vectors overlap the reference coordinate axis of the gyro in the same benchmark, the attitude determination accuracy improves.