4, 4'-diaminodiphenyl ether (ODA) and pyromellitic dianhydride (PMDA) were used as monomers. Carbon nanofiber (CNF) was used as the reinforcing agent. CNF reinforced polyimide (PI) composite aerogels were prepared with acidified CNF (a-CNF) via sol-gel process followed by freeze-drying technology. The morphologies, thermal insulation, microwave absorption as well as compression properties of PI composite aerogels were characterized. The results show that the volume of PI composite aerogels is shrunk from 45.52% to 35.32%, and the density is decreased from 0.084 g/cm³ to 0.069 g/cm³ with the increase of a-CNF content. The composite aerogels exhibit bigger pore sizes and wider pore size distribution after the introduction of a-CNF as well. CNF in PI matrix play roles for reducing the shrinkage of PI composite aerogels, thereby the thermal conductivity is reduced. Additionally, the reflection loss (RL) of PI composite aerogel with 15%(mass fraction) of a-CNF (15% CNF/PI) reaches -9.7 dB at 8.3 GHz. This is due to the fact that the introduction of CNF induced the conduction loss and the porous structure of aerogels provides better impedance matching. The compressive strength and modulus of PI composite aerogels with 15% of a-CNF content are increased by approximately 1.5 times and 2 times compared with pure PI aerogel, respectively.