To explain the moisture adsorption properties and improve the package material of polydextrose product, the adsorption and desorption isotherms of five domestic samples with initial moisture content (IMC) of 1.01%~4.89% were determined in this study in the range of 20~35 ℃ and ERH10%~90% using a dynamic water sorption analyzer. The adsorption and desorption isotherms were sigmoidal-shaped curves, and there was a hysteresis between desorption isotherm and the adsorption isotherm when ERH was less than 80%. Because polydextrose is an amorphous phase powder, we assumed that the water adsorption process of polydextrose was an equilibrium that existed among the vapor phase, the liquid phase (polylayer adsorption) and the solid solution (monolayer adsorption), and the adsorptive and desorptive sigmoidal-shaped isotherms can be de-convolved into a monolayer (Mh) and a polylayer component (Md) in accordance with Hailwood- Horrobin (HH) model. The polylayer water content of adsorption and desorption showed a sigmoidal increase with increasing ERH. The monolayer water content of desorption remained constant with increasing ERH, while that of the adsorption increased slowly for samples a1 to a4 with IMC 4.87% to 2.35% and decreased almost linearly for sample a5 with IMC 1.01%. As the IMCs of the samples decreased, the molecular weight of the polymer at the sorption site was maximum for sample a1 and minimum for sample a5. Although the FTIR showed the smallest intensity of hydroxyl group (3 416 cm–1) and amorphous structural functional group (1 027 cm–1) for a5 sample, a5 sample had the largest moisture absorption rate. These results suggested that, for the lower moisture content of polydextrose samples processed by different processes, their adsorptive monolayer water content seemed to decrease almost linearly with an increase of ERH. However their adsorptive polylayer water content sigmoidally increased with an increase of ERH, and their moisture absorption rate and hygroscopic capacity increased.