An operating method of microwave heating device is provided, in which a holder is disposed in a heating chamber, and a plurality of microwave transmitters are arranged outside the heating chamber. A plurality of half-wave-rectified power supplies are provided to connect the microwave transmitters, and the half-wave-rectified power supplies have capacitances respectively. A plurality of longitudinal waveguides and a plurality of transverse waveguides are installed in between the heating chamber and the microwave transmitters. The capacitance of each of the capacitors of the half-wave-rectified power supplies is adjusted, such that the microwave power pulse bandwidth of the microwave transmitters are extended to produce a plurality of overlapped couplings. The half-wave-rectified power supplies supply power to the microwave transmitters, so that the microwaves are guided into the heating chamber by the longitudinal waveguides and the transverse waveguides for exciting multiple microwave modes in the heating chamber.