Simultaneous improvement of mechanical and microwave absorption properties of the composites at high temperatures still undergoes considerable challenges. We have investigated the high-temperature microwave absorbing properties of the silicon carbide fiber-reinforced oxide matrices (SiCf/mullite-SiO2) composite on the basis of our previous work. Results indicate that the complex permittivity increases from 8.19 - j5.09 to 16.39 - j9.83 at 10 GHz with the temperature rising from 200 to 600 degrees C. The SiCf /mullite-SiO2 composite has relatively high tan delta values indicating superior microwave attenuation ability. The reflection loss (RL) values of the composite increase with rising thickness. It can be noticed that the RL response curves of different thicknesses are basically consistent at 200 and 400 degrees C. In addition, the RL value of the composite is less than - 5 dB in the whole X band when the thickness is under 2.9 mm and the temperature is below 400 degrees C. The hybrid oxide matrices of mullite and SiO2 are beneficial to improve the dielectric properties, especially high-temperature microwave absorption properties of the SiC fiber-reinforced ceramic matrix composite. The superior microwave absorption properties indicate that the SiCf/mullite-SiO2 composite is a promising candidate in aircraft engine nozzle and aerodynamic heating parts of aircrafts at high temperatures.