The invention relates to a gas-fired boiler for overheating foreign steam. The gas-fired boiler comprises a pot part composed of a steam pocket, a front forced-circulation evaporator, a composite overheater, a rear natural circulation evaporator and a coal economizer, and a furnace part composed of a combustor, a combustion chamber, an air preheater, a gas preheater, a furnace body and a chimney. The combustor is arranged at the front end of the furnace body, the chimney is arranged at the tail end of the furnace body, the front forced-circulation evaporator, the composite overheater, the rear natural circulation evaporator and the coal economizer are sequentially arranged in the furnace body from front to back, and the steam pocket is arranged above the outside of the furnace body. Middle-pressure steam generated by the gas-fired boiler and foreign low-pressure steam are mixed in a steam ejector, then enter the composite overheater, and are overheated to the needed overheating temperature, the steam can adapt to the change of the load of the foreign steam, continuous stable high-quality overheated steam is provided for a steam turbine generator unit for power generation or other heat users, efficient utilization of energy is achieved, and the aims of energy saving and emission reduction are achieved.