Polyacrylonitrile copolymer (CPAN) was obtained by water-deposit polymerization, using acrylonitrile (AN), vinyl acetate (VAc), potential crosslinking moiety (hydropropyl acrylate, HQ) as monomer, $NaClO_3$ as initiator. CPAN was blended with cellulose acetate (CA) to prepare CPAN/CA blending fiber via wet spinning. The fiber was post-crosslinked and hydrolyzed to get water absorbency fiber. The structures and properties of the fibers were studied and the results showed that potential crosslinking moiety was necessary in making CPAN/CA blending water absorbency fiber. The saponification reaction was accelerated after blended with CA and the interfacial microvoids enhanced the water absorbency ability.