By referring to the BET polymolecular layer adsorption model, this paper has built up the polymolecular layer adsorption model for the moisture adsorption of fabrics. For polyester— cotton fabrics of different warp and weft densities and different weave structures, in this paper we have carried out moisture adsorption experiments at 20—45°C and within a relative humidity range of 20—95%. Based on the experimental data of equilibrium dry-basis water content of moisture adsorption, in this paper we estimate the value of three parameters of the BET model with Mathematica software, and obtain the numerical value of each parameter in the model, namely the number of moisture adsorption layers in fabric (n), the moisture adsorption performance parameter of materials (C), the saturated equilibrium adsorption dry-basis water content (Ym ), and then the equilibrium dry-basis water content of materials after moisture adsorption under a certain temperature and relative humidity can be calculated from this model. According to the result, the simulated value tallies well with the experimental value and this shows that polymolecular layer adsorption theory could describe the moisture adsorption mechanism of fabrics well. In addition, we compare and analyze the differences in moisture adsorption performance among fabrics of different warp and weft densities and different weave structures.