Abstract: The surface of short-cut p-aramid fibers was modified with chlorosulfonic acid to introduce —SO2Cl groups. An optimal modification effect was achieved at a chlorosulfonic acid concentration of 40 mmol/L. The fibers were modified by using phenyltriethoxysilane as a coupling agent， and the coupling agent was successfully grafted onto the fiber surface. The polyacrylonitrile/p-aramid fiber composites were prepared through in-situ polymerization of modified p-aramid fiber with benzonitrile monomer. The effect of fiber content on the mechanical properties of the composites was investigated. The composite obtained optimal mechanical properties at a fiber content of 0.5 wt%， and its storage modulus， glass transition temperature， and bending modulus reached 4，137 MPa， 350 ℃， and 4，467 MPa， respectively.

Key words: chopped para-aramid fiber, sulfonated modification, phthalonitrile resin, mechanical property