The invention discloses an automatic adjustment method and device of a boiling water capacity of a heat accumulating type water boiler. The automatic adjustment method comprises the steps that first water level information and a plurality of control water levels are acquired, wherein the first water level information comprises a first maximum water level and a first heating starting water level; when a situation that a current water level drops to the first heating starting water level is detected, a heating water replenishing procedure is started; whether the water level is less than the first heating starting water level is judged; if the water level is less than the first heating starting water level, water level variation information is recorded according to a control water level parameter; and second water level information is determined according to the water level variation information, wherein the second water level information comprises a second maximum water level and a second heating starting water level. Through the automatic adjustment method and device, the mode that the hot water storage capacity of the water boiler is quantitative is changed, the effect that the boiling water storage capacity of the water boiler is automatically adjusted according to different numbers of people fetching water is realized, the water supply quality is improved, and loss of heat isreduced to the greatest extent.