Abstract: In this paper,the factors of moisture absorption and moisture permeability of fiber assembly were defined according to its mechanism.And a model was stated.With the original simulator named Shape-Ganerator(SG) which was compiled by Visual C++,the moisture absorption and moisture permeability of PTT fiber assembly with different linear density was simulated on computer.The result of simulation indicates that moisture absorption of PTT fiber assembly was improved along with fining of fibers till a certain size.When the linear density of fiber is 0.36 dtex,the moisture absorption of fiber assembly comes to extremeness.And further fining of fibers is ineffectual for moisture permeability.In the other hand,moisture permeability of PTT fiber assembly was declined along with fining of fibers.