The invention discloses a device for detecting isothermal adsorption and desorption of the moisture of a tobacco slice or cut tobacco biomass. The device consists of a gas mixing unit, an adsorption and desorption unit, a weighing unit and a data acquisition and control unit, wherein the gas mixing unit and the adsorption and desorption unit are respectively arranged independently. By adopting the device disclosed by the invention, a process of performing isothermal adsorption and desorption on the moisture in tobacco slices or cut tobaccos can be detected under certain temperature and humidity conditions; the moisture adsorption and desorption rates of different samples and moisture content results calculated in an equilibrium state can be acquired by analyzing the change conditions of the moisture in the samples along with the time so as to ensure that the moisture adsorption and desorption performance of biomass materials such as the tobacco slices or the cut tobaccos can be represented. The device disclosed by the invention has the advantages that an operating temperature and humidity range is wide; an adjustment range of the temperature and humidity is that the temperature is 5-90 DEG C and the relative humidity is 5-95%; and moreover, the stability of a weighing sensor under high-temperature test conditions can be ensured, and adsorption and desorption tests of the biomass materials in high-temperature environments can be achieved.