With an increasing scale of the photovoltaic power station in the power grid, the stability and security of power grids have become an important concern in recent years. Online monitoring of photovoltaic power stations, which can reflect the station operation status in real time, provide the necessary support for its operation and routine maintenance, and meanwhile effectively improve its operational stability. This paper reviews the existing researches from three aspects of power monitoring, environmental monitoring and fault online monitoring. The power monitoring mainly includes the power control strategy and short-term power prediction technologies, while the environmental monitoring focuses on tracking temperature, light intensity and surface cleanliness of battery panel. Two mechanisms in online fault diagnosis technology, non-intelligent and intelligent, are discussed. Finally, common communication approaches in online monitoring system of photovoltaic power station are compared, and their characteristics are emphatically analyzed. It is predicted that the intelligent, standardized fusion with multi- mechanism and multi-parameter is becoming a major development trend for future online monitoring technologies.