The proper determination of high-temperature constitutive properties and damage of polymer-matrix composites (PMC) in an aggressive environment is critical in high-speed aircraft and propulsion material development, structural integrity, and long-term life prediction. In this paper, a computational micromechanics study is conducted to obtain high-temperature constitutive properties of the PMC undergoing simultaneous thermal oxidation reaction, microstructural damage, and thermomechanical loading. The computational micromechanics approach follows the recently developed irreversible thermodynamic theory for polymer composites with reaction and microstructural change under combined chemical, thermal, and mechanical loading. Proper microstructural modeling of the PMC is presented to ensure that reaction activities, thermal and mechanical responses of the matrix, fibers, and fiber-matrix interface are fully addressed. A multiscale homogenization theory is used in conjunction with a finite element representation of material and reaction details to determine continuous evolution of composite microstructure change and associated degradation of the mechanical and physical properties. Numerical examples are given on a commonly used G30-500/PMR15 composite for illustration.