It is found that ablation and carbonization of GFRP irradiated by laser will induce microwave transmission decay since some char products appear. To study this phenomenon, a numerical simulation investigation was carried out, which was considered composing of laser irradiating, material thermal response, characteristics quantity modeling, and microwave transmission decay analysis. A temperature calculation numerical model of GFRP laminated plate irradiated by laser was built and validated by measuring its laser coupling characteristics and surface temperature. By analyzing the temporal evolution process of GFRP volumetric temperature field, two model characteristics quantities were built: weighted sum of duration time of mesh elements temperature exceed-ing threshold temperature T_c, described as S_{t, Tc}, and weighted sum of product of temperature and duration time of mesh elements temperature exceeding threshold temperature, described as S_{Tt, Tc}. The experiment and numerical simulation results of microwave transmission decay were analyzed, applying a method which calibrated the model characteristics quantities by a selected experimental result, and analyzed matching deviation of the entire data. It is found that, S_{t, Tc} is well characterizing the microwave transmission decay effect of GFRP rather than S_{Tt, Tc}, and the best matching result of R² is 0.9956 when the temperature threshold is 873 ℃. It is demonstrated that the model can simulate and predict the microwave transmission decay by calculating the temperature response of GFRP irradiated by laser and distilling S_{t, Tc} from the volumetric temperature temporal evolution.