Phase-locked loop (PLL) plays an important role in wireless communication navigation, radar and other fields. It affects the data processing rate of the whole system and the extraction of clock phase information. The optical phase-locked loop (OPLL) has the advantages of high-speed real-time reception, low power consumption, strong ability to resist background light interference and Doppler frequency shift, and is the key circuit module of coherent optical communication system. Its performance directly affects the performance of optical communication system. The research of optical phase-locked loop is mainly aimed at optimizing and improving its parameters. This paper reviews the research progress of the technology from electric PLL to OPLL. Based on an introduction to the principle and key components, the classification and parameters of the electric PLL and optical PLL are introduced in detail, and the performance of various optical PLLs is compared. In addition, the OPLL technology applied in coherent optical wireless communication system is summarized, and the future development direction is prospected, which provides a benchmark for research and development of PLL technology in different areas.