In the photovoltaic(PV) plant, the parasitic capacitance between the PV panel and the ground(CPV) causes leakage current in the non-isolated systems. The case can be deteriorated in the rainy environment because the CPV increases dramatically due to the rainwater. The existing off-line methods are either through simulation or measurement by impedance analyzer, which are not easy to conduct on-site especially in the PV plant when it rains. The existing on-line capacitance measurement method is to measure the amplitude of the main components of voltage and current on the capacitance, but the measurement results are inaccurate because the parasitic resistance and inductance of the PV panels are not considered. This paper presents a simple on-line extraction method for the CPV, which is obtained through measurement of the oscillation period of the leakage current. In this way, the CPV can be obtained even the equipment is running. The principle is analyzed in a full-bridge case and the simulation and experiment are carried out for the verification. Finally, the CPV obtained in this method is compared with the 3D finite element simulation, the measurement by impedance analyzer and the existing on-line calculation method, which shows the accuracy of this proposed method.