The invention discloses a gas water heater. The gas water heater comprises a heat exchanger, a combustor, a controller, a water inlet end and a water outlet end, wherein the water outlet end is connected with the water inlet end, a main pipeline and a bypass pipeline are connected between the water inlet end and the heat exchanger in parallel, a water pump and a first water flow sensor are arranged on the main pipeline, a second water flow sensor is arranged on the bypass pipeline, a water storage tank is arranged between the water outlet end and the heat exchanger and is provided with a water storage cavity and a coil pipe arranged in the water storage cavity, a first branch and a second branch which are arranged in parallel are arranged between the water storage tank and the heat exchanger, and a third branch and a fourth branch which are arranged in parallel are arranged between the water storage tank and the water outlet end. The gas water heater disclosed by the invention is provided with the water pump and the water storage tank in a built-in manner, and the free switchover among a preheating mode, a bathing mode and a heat insulation mode is realized through waterway switching; no cold water can be discharged from the water outlet end, and meanwhile, the phenomenon that water of the water outlet end is sometimes hot and sometimes cold due to multipoint water supplying is avoided.