Polyimide (PI) fibers pose potential problems in applications. Their low surface activity causes poor interfacial wettability, easy agglomeration in aqueous solutions, and poor dispersibility. Therefore, this work proposes a method of surface modification of alkali-treated PI fibers with cellulose nanocrystals (CNCs) under the combined catalytic action of a Lewis acid and a crosslinker. The dispersion degree of PI fibers in aqueous solution before and after CNC modification and the contact angle of the PI fiber paper are measured. The results show that the wettability of the PI fibers improved. Furthermore, the structure and properties of PI fibers before and after CNC treatment are characterized via scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis. The pore-size distribution of the PI-fiber paper is measured by a porous-material pore-size analyzer. Compared with the original PI fibers, the oxygen content of the fiber surfaces increases after CNC treatment because of the esterification reaction and crosslinking that occur on the surfaces. The increase in the number of oxygen-containing polar groups and the increased surface roughness of the PI fiber improve its wettability. The contact angle of the PI fiber paper in deionized water is reduced by 14.9 degrees and that in ethanol by 4.8 degrees; the fiber dispersion degree is increased by 45%. These results indicate that the fibers have remarkably improved hydrophilicity and dispersion in the aqueous phase. Therefore, the method developed herein is to prepare high-performance organic fibers and corresponding composite materials.