Abstract: Mori-Tanaka method and the modified shear-lag model were combined to develop formulae to predict the modulus and to give the stress field of unidirectional short fiber reinforced high density foam plastics. The finite element models accounting for different cases were also built to analyze the stress distribution of the fibers and matrix. The research results show that the theoretical prediction agrees well with the solution of the finite element analysis. The modified shear-lag model can be used to explain the stress transfer mechanism of the material. It is found that the axial stress of fibers and interfacial shear stress increase with increasing the volume fraction of void，which would make fibers easier to debond and break. The situations，fiber debonded at the end or penetrated void，lead to local stress concentration and have small influence on the whole stress distribution of the material.