After the sludge is dried, the hot and humid air is cooled and dehumidified by the heat pump heat exchanger, the sensible heat of the hot and humid air and the condensation of water vapor are recovered to release the latent heat, and then the heat pump heat exchanger is used to heat the dry and cold air into dry hot air, and the dry hot air passes through the sludge layer. The flow heat transfer is hot and humid air, and the hot and humid air enters the lower circulation, which can not only dry and dehumidify the sludge, but also achieve the purpose of energy saving.