There are many factors that affect the power generation capacity of photovoltaic power generation system. By measuring the attenuation rate of photovoltaic modules, we can guide the design and construction of photovoltaic power station in the later stage, and improve the overall power generation capacity of photovoltaic power station. In view of the shortcomings of the existing PV module decay rate test methods, this paper proposes a new method for on-line measurement of PV module attenuation rate based on the in-depth mining and analysis of a large number of long-term monitoring data based on the on-line test device in the demonstration base. This method has the advantages of meeting the actual working conditions of photovoltaic modules, continuously measuring a large amount of data, proposing data acquisition rules and data selection boundary conditions to ensure the validity and representativeness of data. By comparing the theoretical and experimental attenuation rates of different types of photovoltaic modules, the deviation is less than 0.04%. The results show that the design of the measuring method is reasonable, effective and feasible.