A mechanical vapour-recompression low-temperature air drier and a method thereof belong to the technical field of energy conservation. The mechanical vapour-recompression low-temperature air drier is characterized by comprising a material drying tube (1), a compressor (3), a motor (2), an expansion machine (5), an air preheater (4), a gas-liquid separator (6) and a draining pump (7), wherein the air preheater (4) is embedded in the material drying tube (1). The material drying tube (1) is sequentially connected with the compressor (3), a hot fluid inlet of the air preheater (4), and an inlet of the gas-liquid separator (6); the gas-liquid separator (6) is provided with a gas phase outlet and a liquid phase outlet, and the liquid phase outlet of the gas-liquid separator (6) is connected with the draining pump (7); wet air at the gas-phase outlet of the gas-liquid separator (6) is mixed with a raw material and then enters into the material drying tube (1); and the drive motor (2) is connected with the compressor (3) and a rotor of the expansion machine (5). The mechanical vapour-recompression low-temperature air drying process has the characteristics of being high in drying rate, low in energy consumption and wide in drying temperature application range, and is particularly suitable for transformation of energy conservation and emission reduction on a clothes drying process.