The invention discloses a normal-pressure horizontal fixed fire grate gasification layer firing boiler, wherein the upper part of the boiler is provided with a flat top plate, corners are in circular R structures, the surface of the boiler is military green, the front surface, the back surface and both sides of a boiler body of the boiler are vertical surfaces, and the boiler body is in a horizontal square structure; the bottom of the boiler is provided with two bases, two fixed fire grates formed by respectively connecting two ends of a plurality of water tubes in parallel are arranged above the bases, the upper parts of the two fire grates are respectively provided with two combustion chambers, the upper parts of the two combustion chambers are respectively provided with larynx openings and are commonly provided with one boiler furnace, the middle of the boiler furnace is provided with a refraction arch, the upper part of the boiler furnace is provided with a front convection pipe bundle, the back side of the boiler furnace is provided with one smoke inlet entering the boiler back body, and the upper part of the middle part of the boiler back body is vertically provided with a vertical table flap downwards. The invention is a boiler with double combustion chambers, has the characteristics of high efficiency, energy saving, environment protection, security, reliability, scientific design, small size and low operation cost and is applied to offices, schools, hotels and bath centers.