The effect of elastic and thermal mismatches between fiber and matrix on the push-in mechanism and interfacial shear strength () measurement of typical polymer and ceramic matrix composites was studied in this work by comprehensive finite element modeling of push-in response, using carbon fiber-reinforced epoxy and SiC matrix (C_f/epoxy and C_f/SiC) as model materials. The results highlight a strong effect of fiber-localized environment on the push-in response, especially for the C_f/epoxy with large elastic mismatch. This led to errors to estimate if applying directly the standard shear-lag model to calculate τ₀ according to the push-in response. Calibrations of the shear-lag model were thus performed in both C_f/epoxy and C_f/SiC, and the results suggest a negligible effect of the elastic mismatch in the C_f/SiC due to the comparable elastic properties between fiber and matrix. The shear-lag model can thus well estimate based on the push-in force–displacement curves. If the residual thermal stress was considered, the push-in deformation was altered by increasing evidently the critical load leading to interfacial fracture, which led to overestimation of . This is a consequence of the large shear stress level concentrated mainly at the fiber/matrix interface. Detailed parametric push-in simulations were afterwards performed to incorporate the effect of interfacial shear stress in the standard shear-lag model, and a more proper shear-lag model calibrated by incorporating both elastic and thermal mismatches was finally proposed.