The invention discloses a heat exchanger of a gas-fired boiler. The heat exchanger of the gas-fired boiler comprises a barrel, a motor, a stirring paddle and a temperature sensor, wherein the motor is installed in the middle of the top end of the barrel; the stirring paddle is arranged inside the barrel and connected with the motor through a rotary rod; the temperature sensor is installed on the inner wall of the barrel; a water inlet and a gas outlet are formed in the top end of the barrel; and a gas inlet and a water outlet are formed in the bottom end of the barrel. The heat exchanger of the gas-fired boiler achieves sufficient absorption of waste gas heat and avoids heat and fuel waste.