The invention discloses a commutation overvoltage suppressor for improving the reliability of a commutation valve, which is composed of a variable reactor and a resistor. The variable reactor comprises an iron core on which a main inductance coil and a bias coil are wound; One end of the main inductance coil is connected with the valve side bus bar of the rheostat, and the other end is connected with the resistor in series and grounded; The bias winding is connected with a variable DC current source. A method for adjust inductance includes such step as calculating initial stray parameter to ground, calculating reactance value of overvoltage suppressor, adjusting inductance accord to system parameter and commutation overvoltage monitoring value in operation, and adjusting inductance value according to overvoltage monitoring value. Through the effect of commutation overvoltage suppressor, the stray capacitance produced by valve side bushing of converter can be eliminated, and the voltagestress of lightning arrester and thyristor of converter valve can be effectively reduced, so as to reduce the risk of failure and frequent maintenance of converter valve.