An electronic transformer and a microwave cooking appliance. The electronic transformer includes a rectifier device, a transformer, a switch device, and a control device. The rectifier device is connected to an alternating current source; the transformer is connected to the rectifier device; the switch device is configured to provide an on/off signal to the transformer; the control device is connected to the switch device; the control device is configured to generate a control signal according to preset power and provide the control signal to the switch device to control the switching frequency of the switch device.