A power converter and a modulation method for a three-phase four-bridge-arm inverter circuit are disclosed. An inverter includes the three-phase four-bridge-arm inverter circuit and a controller. The controller is configured to: when an absolute value of an instantaneous value of a modulation signal of a bridge arm N is greater than a first effective modulus value, inject a common mode signal into the bridge arm N in a unipolar manner; or when an absolute value of an instantaneous value of a modulation signal of a bridge arm N is less than a second effective modulus value, inject a common mode signal is into the bridge arm N in a bipolar manner. This can prevent a loss of a modulation signal of the bridge arm N, reduce a switching loss of a main bridge arm, and improve quality of electric energy output by the power converter.