The invention discloses a control method of gas water heater and a gas water heater. The control method comprises the following steps that when the gas water heater is in operation, and the environmental oxygen concentration of the gas water heater is collected; judging whether the output value of a valve body is equal to the preset output value under the condition that the environment oxygen concentration is in a preset range; the preset output value is related to a preset range; under the condition that the output value of the valve body is not equal to the preset output value, and the output valve of the valve body is adjusted according to the preset output value. According to the control method of the gas water heater, the output value of the valve body is adjusted by collecting the environment oxygen concentration of the gas water heater, so that the operation of the gas water heater is more suitable for the environment, so that the gas water heater can be accurately controlled, and the problem of poor system control deviation of the gas water heater can be avoided or reduced, so that the problems of incomplete combustion, vibration combustion, high waste gas emission and thelike of the gas water heater can be solved.