The precursor impregnation pyrolysis process （PIP） was a near-net-shape preparation method for SiC/SiC composites， but the stress generated during the precursor pyrolysis process hindered the industrialization of the process. In this study， a chemical-mechanical couple model was proposed to study the mechanism of process stress and simulate the stress evolution during pyrolysis. The pyrolysis process of precursor was described by a kinetic model. An analytical model derived from the three-phase sphere model was developed to determine the homogenized properties and behavior of matrix. A finite element model integrating the kinetic model and the analytical model was established to compute the process stress of a microscale representative volume cell （RVC）. The results show that the matrix sustains significant tensile circumferential stress at the interface of matrix-poor area. For the matrix composed by pure precursor， the evolution of the process stress is dominated by the chemical shrink due to the pyrolysis process and slightly influenced by the thermal expansion due to the process temperature. With the increasing of PIP cycle numbers， the roles of chemical shrink and of thermal expansion inverse.