The invention discloses an on-line testing device for the thermophysical property of a cut tobacco in a drum drying process. The on-line testing device is characterized by being suitable for being mounted in a miniature drum drying device to be used and comprising a cut tobacco sampling unit, a thermophysical property testing unit and an on-line temperature measuring unit. An on-line testing method comprises the following steps of: sampling the cut tobacco which is dried to be in one specific state (a water content, a temperature) and is in a natural accumulation state through the cut tobacco sampling unit, and measuring the heat conductivity coefficient, the heat diffusion coefficient and the volumetric heat capacity of the cut tobacco by utilizing the thermophysical property testing unit. Compared with the prior art, the on-line testing device has the following advantage that on-line measurement for the temperature, the heat conductivity coefficient, the heat diffusion coefficient and the volumetric heat capacity of the cut tobacco in the natural accumulation state can be realized. The testing time is shorter, and the repeatability and the stability of data are good.