Double IMU/DGPS integrated attitude measurement system is used for measuring the relative attitude of two points on a moving carrier. The system integrates the velocity and position information of DGPS with the navigation information of the master INS and the slave INS respectively, and the relative attitude can be calculated. In order to realize the synchronization of DGPS and INS as well as the two INS, the method adopting quartz oscillator in INS as the time reference was proposed. The hardware-in-the-loop simulation results show that the synchronization error of the INS decreases from 10ms down to 100ns by the proposed method. The synchronization precision meets the requirement and the method has high reliability.