The invention provides a condensing boiler which comprises a combustor, a combustion chamber, a condensing heat exchange barrel, an inner circulating system and an outer circulating system. The combustion chamber is disposed in the condensing heat exchange barrel, a connecting pipeline on the side wall of the combustion chamber penetrates out of the condensing heat exchange barrel and is used for being connected with the combustor, a top barrel wall of the combustion chamber is connected with a smoke diversion elbow, a smoke outgoing port of the smoke diversion elbow is formed downwards, and a boiler inner circulating spray device, a water feeding spray device and an air preheating system are arranged above the smoke diversion elbow. The inner circulating system is characterized in that boiler water is circulated into the inner circulating spray device above the combustion chamber in a forced manner, inner circulation of the boiler water is enhanced, and heat is absorbed better. Through a special arrangement mode of the combustor, the smoke diversion elbow, the spray devices, a demisting and preheating device and the inner and outer circulating systems, so that the condensing boiler is more convenient and quicker to mount, direct contact of high-temperature smoke with boiler feed water for heat exchange can be efficiently realized, the temperature of discharged smoke can be lowered fully, and heat efficiency of the boiler can be improved fully.