The invention relates to the technical field of boiler structures, in particular to a gas-fired boiler. After the gas-fired boiler is remolded, the performance parameters of an original CFB boiler are retained, and natural combustion of fuel gas is guaranteed. A gas burner is mounted at the bottom of a hearth. An SOFA opening is formed in the side wall of the hearth. The upper end of the hearth communicates with a connecting flue. The connecting flue comprises a horizontal flue body, a vertical ascending flue body and a vertical descending flue body. The vertical descending flue body communicates with a tail flue body. A high-temperature superheater, lower-temperature superheaters, economizers and two sets of air preheaters are sequentially mounted in the tail flue body. Each economizer is formed by spiral fin pipes. A cold air inlet is formed in the lower air preheater. A hot air outlet is formed in the upper air preheater.