The invention discloses a steam boiler with a pressure blowdown thorough-cleaning function. The steam boiler with the pressure blowdown thorough-cleaning function comprises a furnace body and a waterdischarging pipe arranged at the bottom of the furnace body, wherein the water discharging pipe is a rigid pipeline, pipe diameters of all sections of the water discharging pipe are equal, the water discharging pipe communicates with a thick pipe of which the pipe diameter is larger than that of the water discharging pipe, a vertical pipe communicates with the side wall of the water discharging pipe, a piston plate is connected in the vertical pipe in a sealing and sliding mode, a flat and hollow heat conduction box is fixedly connected to the piston plate through a connecting rod, and one endof the heat conduction box communicates with the water discharging pipe through a first hose. The steam boiler with the pressure blowdown thorough-cleaning function has the advantages that the waterpressure is large in a boiler water discharging initial stage, the water pressure in the water discharging pipe enables the heat conduction box to move upwards to make contact with a fluid pipe, and arotary mechanism is driven to rotate and compress a torsion spring; and when the water pressure in the boiler is reduced, the torsion spring drives a knocking device to rotate reversely, when reverserotation of the knocking device is accelerated, a limiting block is separated from a limiting groove under the acceleration action of the knocking device so that a collision block impacts the furnacebody, then residual dirt in the furnace body can be knocked down, and the effect that the dirt in the boiler is treated more thoroughly is achieved.