Abstract: A gas-fired steam-injection boiler has a flue and a water supply pipeline. The flue is provided with a radiation section and a convection section. The water supply pipeline has, in the flue gas flow direction, a first pipe section located at an upstream portion of the convection section, a second pipe section is located at a downstream portion of the convection section, and a third pipe section arranged between the first pipe section and the second pipe section. In a water flow direction of the water supply pipeline, the second pipe section, the first pipe section and the third pipe section are arranged in sequence. By using a flue gas condenser, the temperature of flue gas can be further reduced, and the latent heat of vaporization of steam in the flue gas is absorbed to form condensate water.