The fixed grate biomass boiler features a fixed grate design, with dual-directional air supply along the width of the grate. The grate is divided into seven air chambers on each side, front and rear, each equipped with an air volume adjustment door at the inlet. Above the grate is the boiler furnace, with water-cooled walls surrounding it. In the horizontal flue at the furnace outlet, a high-temperature superheater and a low-temperature superheater are sequentially arranged, with superheated steam temperature regulated by a water spray desuperheater. At the rear vertical shaft, an economizer and a tubular air preheater are arranged along the flue gas flow direction.