The invention discloses a quick heating device of a water heater. The quick heating device comprises a microwave heating chamber and pressure relief air sacs, wherein a water storage bottle is mounted in a heating cavity of the microwave heating chamber; the upper end of the water storage bottle communicates with pressure relief pipes; the lower ends of the pressure relief air sacs sleeve the upper ends of the pressure relief pipes in a sealed and fixed manner; explosion-proof enclosures sleeve the pressure relief air sacs; the lower ends of the explosion-proof enclosures are fixedly connected on the surface of the upper part of the microwave heating chamber; the two sides of the water storage bottle separately communicate with a water inlet pipe and a water outlet pipe which penetrate through the side wall of the microwave heating chamber. According to the quick heating device, the microwave heating chamber can perform quick heating, the pressure relief air sacs can relieve pressure of water vapor generated from blasting boiling in the water storage bottle during microwave heating, explosion is prevented, safety is guaranteed, and service life of the water heater is prolonged; and microwave heating and electric heating are combined, the water heater can achieve quick heating, water temperature is stable, safety utilization performance is achieved, and the quick heating device deserves popularization.