A gas-fired steam boiler is a steam boiler that utilizes gas combustion for heating. The vertical steam boiler employs a lower-mounted burner and a two-pass structure, ensuring complete fuel combustion, stable operation, and minimal space occupancy. Additionally, baffles are inserted into the flue tubes to slow down exhaust gas velocity and increase heat exchange, resulting in high boiler thermal efficiency and reduced operational costs for users. The horizontal steam boiler features a shell-type, fully wet-back, downstream three-pass fire tube structure. The flame burns micro-positively within the large combustion chamber, fully extending for low heat load and high thermal efficiency. It effectively reduces exhaust gas temperature, promotes energy conservation and cost reduction, and is more economical. It also uses a corrugated furnace tube and threaded flue tube structure, which enhances the boiler's heat absorption strength while meeting the heat expansion needs of the heat exchange surface, making it scientifically rational and durable.