Abstract: In order to reveal the moisture absorption mechanism of ultra-high molecular weight polyethylene (UHMWPE) fiber-reinforced epoxy composites, the two-dimensional finite element models were established by using ABAQUS, and the hygroscopic behavior and moisture-induced stress in the composites were studied. The moisture concentration fields in two models with different fiber distributions and environment temperatures were calculated, and then the moisture-induced stresses were analyzed on the basis of the first step. The results show that the moisture absorption processes of the two models all are in agreement with Fick’s diffusion law, the saturation time of moisture absorption of the model with hexagonally distributed fibers is less than that of the model with randomly distributed fibers, but the latter is more coincidental with practical situation and fitted well with the experimental data. The stress within the materials induced by long time moisture absorption can reach to a higher level (above 60 MPa), the higher the temperature, the more early the moisture absorption balance, the greater the stress, and the maximal value appears in the matrix region where fiber aggregation state is most crowded. The stress level of the model with randomly distributed fibers is obviously higher than that of the model with hexagonally distributed fibers.