An air-drying device for sludge comprises at least one air-drying mechanism, a dosing mechanism, and a dehumidifying and heating mechanism. There two air-drying mechanisms. The dosing mechanism connects with the beginning end of the first air-drying mechanism (5) whose terminal end communicates with the beginning end of the second air-drying mechanism (8), with a discharge port (21) is arranged above the communicating position. The terminal end of the second air-drying mechanism communicates with the beginning end of the first air-drying mechanism to form a loop configuration. The device has high efficiency and low energy consumption. The detention time of the sludge in the drying device is adjustable so that the water content of the discharging material is adjustable.