Based on the leakage current of capacitive equipments， a measurement method of power voltage and internal over-voltage is presented. A high precision and wide-band current sensor is used to detect the leakage current of a capacitive equipment. The current signal is converted into voltage signal by an active integrator. The equivalent circuit of the capacitive equipment and the transfer function between voltage and leakage current are analyzed. The principle and influential factors on the current sensor performance are also studied. By using the active integrator， the amplitude-frequency characteristic of the output voltage is improved. The adopted voltage sensor is very simple in structure and very safe because it has no direct connection with the high voltage system. Experimental results of power frequency voltage and switching over-voltage indicate that the voltage sensor has very good precision and linearity， and can be used to measure the voltage of high voltage power grids.