The microwave transmittance of glass fiber-reinforced plastic (GFRP) slabs subjected to continuous-wave laser ablation was studied in the framework of continuum mechanics. First, a one-dimensional physical model involving laser absorption, heat conduction, resin pyrolysis, thermal radiation, and convection heat transfer was established to obtain the temperature field. An experiment-based absorption coefficient was proposed to capture the bulk-to-surface absorption transition during laser ablation. Second, the complex dielectric constant was modeled using a solid-state kinetic model describing the graphitization of pyrolysis products. The microwave reflectivity and transmittance were calculated based on the dielectric constant distribution. The agreement of the temperature and microwave transmittance with the experimental results suggests the feasibility of the model. The influence of laser power density, material thickness, and tangential airflow velocity on microwave transmittance was studied based on the model. The microwave transmittance changed nonmonotonically with increasing slab thickness owing to the competition between different physical mechanisms. The existence of tangential airflow reduced the decrease in microwave transmittance, particularly for weaker lasers. This study provides a useful physical model for predicting the microwave transmittance performance of GFRP in extreme heat environments.