Gas water heater

The invention relates to a gas water heater which comprises a heat exchanger, a cold water pipe, a hot water pipe, a water inlet pipe, a water outlet pipe and an automatic constant temperature device. The automatic constant temperature device comprises a tank body, wherein a space for containing water is defined in the tank body; the baffle is arranged in the tank body and divides the space in the tank body into a first subspace and a second subspace which are isolated by fluid; and the driving mechanism is connected with the baffle plate and is configured to promote the baffle plate to move towards a first direction when the water using end starts to use water and promote the baffle plate to move towards a second direction opposite to the first direction when the water using end stops using water. The water inlet pipe communicates with the first subspace, and the hot water pipe and the water outlet pipe communicate with the second subspace. The cold water pipe is configured to be switched from the state of b