An on-line monitoring device based on the capacitance divider for internal and lightning overvoltage in the distribution network is presented. A low damping capacitance-resistance divider is used to transfer the overvoltage signal， and the design principle and response performance of the divider are analyzed. The finite element analysis is used to estimate the amplitude error of the divider， which indicates that the accuracy of the divider can meet the requirement for overvoltage monitoring. In addition， the special data acquisition technique is introduced based on the data double buffer and PCI bus in order to sample a sufficient data length at high sampling rate and reduce the single sampling time. Results of on-the-spot operation show that the device is up to the requirement of overvoltage monitoring.