The carbon fiber/mullite composites (Cf /Mullite) were prepared through compression molding process and solid-phase reaction sintering. The phase composition and microstructure of Cf /Mullite composites were characterized, and the influence of carbon fiber(Cf) content on the dielectric properties and microwave absorption properties of Mullite (3Al2O3 2SiO2) ceramics in the X band (8.2~12.4 GHz) was investigated by using a vector network analyzer. The results showed that Al2O3 and SiO2 fully reacted to produce Mullite ceramics at high temperature, and Cf /Mullite composites ceramics had a relatively dense structure. The dielectric constant and dielectric loss tangent (tan δ) of Cf /Mullite composites increased with the increase of fiber addition, and mullite ceramics exhibited better electromagnetic wave absorption capacity by adding Cf When the Cf content was 1.2% (volume fraction) and Cf /Mullite composites thickness d=1.5 mm, the maximum absorption peak of reflection loss was -33.3 dB and the absorption bandwidth of reflection loss better than -5 dB reached 3.675 GHz, while the absorption bandwidth of reflection loss better than -10 dB reached 2.205 GHz. The addition of Cf obviously improved the electromagnetic wave absorption performance of mullite ceramics.