The dielectric parameters of high-Al aluminum silicate fiberboard (HAF) at 915 MHz and 2 450 MHz and 25-1 000 ℃ were measured by a resonant cavity perturbation method. The power transmission coefficient of HAF was calculated based on an electromagnetic wave transmission line theory, and the transmission performance of HAF at different affecting factors was analyzed. The results show that the dielectric constant and dielectric loss factor of HAF increase at 800 ℃ and the both frequencies. The transmission performance of HAF is different under different polarization modes. Its transmission performance in a transverse magnetic field is better than that in a transverse electric field, and there is the Brewster angle (i.e., 50°-53°), which fully makes the microwave transmitting in transverse magnetic field. The transmission performance of HAF fluctuates with the increase of thickness, and its power transmission coefficient curve has a multiple transmission peak in the thickness range of 0-0.2 m.