Abstract: In order to obtain a microwave absorber with wider absorption band, lighter weight and stronger absorption, PAN/PMMA blend fiber precursor was prepared by wet spinning with polyacrylonitrile（PAN） as carbon precursor and polymethylmethacrylate（PMMA） as porogen. Porous carbon fiber was prepared by pre-oxidation and carbonization. The proportion of PMMA in PAN/PMMA blend fiber affects the pore volume of porous carbon fiber, and then affects the microwave absorption performance. When PAN/PMMA is 70/30, the electromagnetic wave absorption performance of porous carbon fiber is the best, the effective frequency band with reflectivity less than-10 dB is 2.0 GHz, and the minimum reflectivity is-38 dB. Porous carbon fiber not only uses the dielectric loss of solid carbon, but also uses the interference generated by the multiple reflection of electromagnetic wave by the hole to dissipate electromagnetic wave. Therefore, porous carbon fiber is an efficient electromagnetic wave absorber. In addition, the porous carbon fiber prepared at too high carbonization temperature is not conducive to impedance matching due to its high conductivity, which reduces the electromagnetic wave absorption performance. The carbonization temperature of 1 200 ℃ is appropriate.