The combination of gyro and star sensor is used to determine the attitude of the spacecraft. Firstly, the measurement model of gyro and star sensor is established with quaternion as a parameter to describe the attitude of a spacecraft. The filtering state equation with error attitude angle and gyro constant drift error as the state quantity under small deviation is deduced, and the Extended Kalman Filter (EKF )algorithm is used to determine the attitude of the star sensor. Secondly, the simulation results show that the proposed algorithm can achieve high accuracy of attitude determination. Finally, the hardware factors that affect the accuracy of attitude determination and software factors for quantitative data analysis, some meaningful conclusions is achieved, so as to provide theoretical support for engineering practice.