Compared to the traditional Intensity Modulation and Direct Detection (IM/DD) system, the coherent system offers higher capacity and power budget, better meeting the needs of high-capacity Passive Optical Network (PON). In recent years, the application of coherent technology in PON to better support future high-bandwidth services has become a hot research topic. This article summarizes the research progress in coherent PON technology from four aspects: system architecture, coherent simplification, uplink burst-mode detection, and flexible PON. In addition, it proposes laser-shared uplink and downlink Filter Bank Multi-Carrier (FBMC)-PON and Semiconductor Optical Amplifier (SOA) current-adjusted flexible PON schemes.