This paper focused the degradation mechanism of microwave transmittance of glass fiber reinforced polymer (GFRP) plates subjected to laser ablation. During the tests, the wave transmittance experiments were performed on the laser-ablated GFRP plates in Ku- and K-bands (12-26.5 GHz). Based on the experimental results, a thermal-mechanical-electromagnetic numerical model was proposed to achieve the laser ablation and wave transmittance simulation, in which the UMAT and UMATHT user-defined subroutines were developed to analyze the pyrolysis characterization. The results indicated that pyrolysis and ablation products significantly raised the electrical conductivity and dielectric constant, which amplified the impedance mismatch. As the electromagnetic wave transmitted in the ablated the GFRP plate, the greater impedance mismatch promoted the stronger reflection. Furthermore, the pyrolysis char would produce the ohmic loss, which tended to be stable once the depth of the pyrolysis char reached a critical value.