The invention relates to a control method and system for a gas water heater as well as the gas water heater. The control method for the gas water heater comprises the following steps: sensing a practical environment temperature t at which the gas water heater is located; obtaining maximum current Imax at the practical environment temperature t according to a first linear equation, and obtaining minimum current Imin at the practical environment temperature t according to a second linear equation; and performing PWM control on current between Imin and Imax by a main controller to realize controlling opening degree of a proportional valve. The first linear equation represents a linear relationship between the maximum current and the environment temperature if maximal secondary pressure is notchanged, and second linear equation represents a linear relationship between minimum current and the environment temperature if minimum secondary pressure is not changed. Influences, on secondary pressure (namely opening degree) of the proportional valve, of the environment temperature are not considered, and deviation generated as a result of influences, on the secondary pressure of the proportional valve, of the environment temperature is reduced, and heating efficiency of the gas water heater is improved.