The invention discloses an extra-high voltage DC power transmission reactive power control device for suppressing AC transient overvoltage. The extra-high voltage DC power transmission reactive power control device comprises a comparison circuit, a regulation circuit and a selection circuit which are sequentially connected, wherein the comparison circuit is used for comparing AC bus voltages of an extra-high voltage DC power transmission rectification side and an inversion side, selecting the maximum value from the AC bus voltages, and subtracting from a reference voltage to form a voltage difference signal, the regulation circuit is used for converting the voltage difference signal to a control signal, and the selection circuit is used for determining whether the control signal is gated or not. By the extra-high voltage DC power transmission reactive power control device, capacitive reactive power surplus of an AC system can be prevented to cause overvoltage problem when a fault occurs in a DC single-pole line.