In this study, an effective microscale model for fiber-reinforced composites with initial damages is presented to investigate local stress distribution and damage evolution at constitutive material level. To validate the proposed numerical model, experimental data of uniaxial stress–strain responses and off-axis failure strength are employed for a comparison, and a good agreement by comparing with numerical results can be found. To represent the microscale damage evolution in the representative volume element, stiffness degradation coefficients are subtly applied to describe the failure sub-cells. Moreover, microscale damage evolutions and local stress distribution in the composites subjected to uniaxial and biaxial loads are both investigated. The effect of three different modes of initial damage in the composites are studied. The influences of the distribution, location and orientation of initial damage on damage evolutions are also studied.