Abstract: Based on the uplink of the Queqiao communication, navigation, and remote sensing constellation system, it is necessary to insert an Inter-Symbol Guard Time (ISGT) between Pulse Position Modulation (PPM) symbols to avoid the transmission of two consecutive pulses within an extremely short time interval. Based on the frame structure of M PPM + P ISGT and in the presence of signal blockage, an optical communication synchronization method based on timing offset estimation, namely the ISGT timing estimation method, has been proposed to address the issue of photon count detection utilizing the guard time in the existing frame format. This method effectively extracts the timing statistical information carried by the uplink optical signal frame structure, enabling slot synchronization for uplink optical signals. It avoids the additional consumption of data bandwidth caused by pilot signal synchronization while also mitigating or eliminating the impact of noise, such as Earth imagery and environmental stray light, on uplink optical signal synchronization. A joint simulation of this optical communication synchronization method was conducted, and the estimation results of the timing offset verify the feasibility of the ISGT timing estimation method. This approach can provide technical support for the Queqiao communication, navigation, and remote sensing constellation system.