Sometimes it is hard to definitely understand the power generation efficiency as implementing the Photovoltaic modules in field. Thus the promise is not made for the constructor after finishing the PV module installation. In addition, the ageing problem of the PV module leads the decrease of the power generation efficiency as well. Therefore, this study using the Time domain Reflectometry (TDR) as a core technology to develop an approach to evaluate and predict the PV module power generation efficiency in field. The preliminary relationship between the power generation efficiency and TDR impedance measurements is established via the simulated ageing tests. Also the field testing is provided to verify the relationship. Suggestions are revealed to extend the simulated ageing time to further verify the relationship.