With the increasing scale of photovoltaic power station in the power grid year by year, its stability and security are paid more and more attention. Online monitoring can reflect the operation status of photovoltaic power station in real time, provide necessary support for the operation and maintenance of the power station, and effectively improve the operation stability of the power station. This paper reviews the existing research from three aspects: power monitoring, environmental monitoring and fault on-line monitoring. Power monitoring mainly includes power control strategy and short-term power prediction technology; environmental monitoring research mainly focuses on temperature, light intensity and surface cleanliness of battery panel; online fault diagnosis technology includes two mechanisms: non intelligent and intelligent. Finally, the common communication means in online monitoring system are compared, and the characteristics of different methods are analyzed. The intelligent, standardized and multi mechanism fusion multi parameter monitoring system is the development trend of online monitoring technology in the future.