Abstract: A U-HVDC system’s transient overvoltage caused by DC blocking will threaten the safe operation of a system. Thus an analytic formula calculating transient voltage of the commutation bus after DC bipolar blocking is derived. Based on this, the equivalent model of the power network of the sending-end system is established, and the transient overvoltage calculation method of any key node of the AC system is derived. In order to judge the severity of the high voltage crossing problem faced by the new energy node and find the weak link of the system in advance, the impedance ratio is proposed to represent the transient overvoltage degree of the node. The voltage rise value of the node transient voltage increases linearly with the increase of the impedance ratio after DC bipolar blocking. Finally, the CIGRE HVDC standard test system and the Jiquan ±1100 k V DC transmission sending system are set up in the DIg SILENT simulation platform to verify the correctness of the transient overvoltage calculation method of HVDC sending-end system.