To assemble spacecraft automatically and precisely, it is vital to measure the relative spatial pose (position and orientation) of the assembly features of the spacecraft components before assembly. For large-scale spacecraft components, the global measurement method is mainly utilized to guide assembly control, and its accuracy and efficiency have ultimately failed to meet requirements. To address this issue, a novel measurement method is proposed. Since the goal is to measure the relative spatial pose of the assembly features of the spacecraft components, the proposed method measures it directly to ensure the consistency of the measurement and assembly coordinate system. This method has the advantage of high precision because it can reduce the influence of structural parameter errors and is not limited by the scale of the spacecraft components. In addition, it requires only one offline calibration, which significantly improves the efficiency of online measurement and assembly. Taking the control moment gyroscope (CMG) assembly task as an example, a measurement system and its corresponding calibration device are designed and developed. After calibration by the calibration device, the measurement system is mounted on the assembly features of the CMG to measure the relative spatial pose between the assembly features of the CMG and the assembly features of the mounted base (MB). Finally, six assembly experiments are completed according to the measurement results. The experimental results show that this method has high accuracy and can guide the robot to achieve high assembly accuracy, satisfying the assembly requirements of typical spacecraft components.