The invention discloses an automatic heat exchanger used for a gas-fired boiler. The automatic heat exchanger used for the gas-fired boiler comprises a barrel, a top cover and a coiled pipe, wherein the top of the barrel is opened; the top cover is connected with the top end opening of the barrel through a flange plate and a bolt; the coiled pipe is installed inside the barrel, a liquid inlet and a liquid outlet of the coiled pipe are located in the lower portion and the upper portion of the side wall of the barrel correspondingly; a gas inlet is formed in the bottom of the barrel; and a gas outlet is formed in the top cover. The automatic heat exchanger used for the gas-fired boiler can effectively replace heat carried in fuel gas to be waited for being further used, avoid heat and fuel gas waste and improve the water utilization rate and the heat exchange efficiency.