The invention provides a method and a device for controlling a power electronic transformer. The device comprises an acquisition module, a first calculation module, a second calculation module and a PWM (pulse width modulation) module. The device provided by the invention for controlling the power electronic transformer directly aims at each phase d in three-phase voltage, and acquires distribution network voltage u at a first moment and distribution network voltage u at a second moment which is a preset time period 1/(4f) after the first moment. Control for the power electronic transformer can be completed under the condition of not carrying out phase locking, and a control failure caused by a failure in phase locking can be avoided fundamentally.