The present invention provides a water heater, including a combustor, a gas pipe, a combustion controller, a pressure gauge, and a controller. The gas pipe supplies the combustor gas. The combustion controller controls combustion of the combustor. The pressure gauge detects a gas pressure in the gas pipe and generates an electrical signal in association with the gas pressure. The controller is electrically connected to the gas gauge and the combustion controller to receive the electrical signal from the gas gauge and controls the combustion controller according to the electrical signal to adjust the combustion of the combustor. In addition, the present invention further provides a method of controlling the water heater.