An online monitoring device for transient overvoltage of a power distribution network comprises a high-voltage capacitance voltage divider, a secondary resistance capacitance divider, a high-speed data collecting card and a control computer. The high-voltage capacitance voltage divider is connected to a bus and connected with the secondary resistance capacitance divider through a first coaxial cable, the secondary resistance capacitance divider is connected with a high-speed data collecting card through a second coaxial cable, the high-speed data collecting card is connected with the control computer, and the high-voltage capacitance voltage divider, the secondary resistance capacitance divider, the high-speed data collecting card and the control computer are respectively connected with the ground. The highest sampling rate of the online monitoring device can reach to 7.5MS/s in each channel. The online monitoring device can trace and record various kinds of overvoltage of the power distribution network and voltage sag wave shapes of the power distribution network, has various analyzing functions and various wave shape processing functions and is small in power loss and convenient to maintain.