The invention relates to a hot water boiler which comprises a boiler body, a hearth and a water tank. The water tank is arranged in the middle of the boiler body, the water tank and a boiler wall on the left side of the boiler body form left smoke chambers, the water tank and a boiler wall on the right side of the boiler body form right smoke chambers, the hearth is embedded in the lower left corner of the water tank, a plurality of hot air through holes communicated with the left smoke chambers and the right smoke chambers are densely distributed in the water tank, an outlet of the hearth is communicated with the lower portion of the lower-layer right smoke chamber through the hot air through holes formed in the bottom of the water tank, and the lower-layer right smoke chamber, the lower-layer left smoke chamber, the upper-layer right smoke chamber and the upper-layer left smoke chamber are communicated in sequence through the hot air through holes formed in the corresponding positions on the water tank. A dust collecting box communicated with the upper-layer left smoke chamber is arranged on the boiler body, a smoke outlet pipe is arranged on the top wall of the dust collecting box in a penetrating mode, the bottom of the smoke outlet pipe extends to the lower portion of the inner cavity of the dust collecting box, a dust discharge opening is formed in the bottom wall of the dust collecting box, and a dust discharge door is arranged on the dust discharge opening. The hot water boiler has the advantages of being simple in structure, high in heat exchange efficiency and good in dust removing effect.