A preparation method of the fiber of polyacrylonitrile - cellulose acetate, comprising the following steps: 1) mixing 92-94wt% of acrylonitrile monomer with 6-8wt% of vinyl acetate monomer to obtain a mixture, regulating a concentration of the mixture to 30-40wt%, carrying out an aqueous-phase suspension polymerization reaction continuously at a temperature of 58 DEG C to 62 DEG C and at a pH value of 2.5 to 3.5, carrying out a chelation reaction for termination to obtain polymer, removing unreacted monomers, and then carrying out water washing, filtering, granulation shaping and baking to obtain powdery polyacrylonitrile; 2) mixing 65-85wt% of the powdery polyacrylonitrile, 15-35wt% of cellulose acetate and a solvent, and carrying out heating, cooling and filtering to obtain a spinning stock solution; wherein a ratio of the sum of the masses of the powdery polyacrylonitrile and cellulose acetate to the mass of the solvent is (20-25.5):(74.5-80), the solvent is select from a group consisting of dimethylacetamide, dimethyl sulfoxide, dimethylformamide or sodium thiocyanate; and 3) subjecting the spinning stock solution to temperature adjustment and pressure adjustment, carrying out spinning at a temperature of 80 DEG C to 97 DEG C and under a pressure of 0.7MPa to 0.9MPa after filtering, carrying out double-diffusion shaping in a coagulating bath, water washing, drafting, oiling, baking, curling and setting, thereby obtaining the fiber of polyacrylonitrile -cellulose acetate, wherein, the coagulating bath is selected from an aqueous solution of dimethylacetamide, dimethyl sulfoxide, dimethylformamide or sodium thiocyanate and has a concentration of 30-55wt% and a temperature of 25 DEG C to 50 DEG C. 1) mixing 92-94wt% of acrylonitrile monomer with 6-8wt% of vinyl acetate monomer to obtain a mixture, regulating a concentration of the mixture to 30-40wt%, carrying out an aqueous-phase suspension polymerization reaction continuously at a temperature of 58 DEG C to 62 DEG C and at a pH value of 2.5 to 3.5, carrying out a chelation reaction for termination to obtain polymer, removing unreacted monomers, and then carrying out water washing, filtering, granulation shaping and baking to obtain powdery polyacrylonitrile; 2) mixing 65-85wt% of the powdery polyacrylonitrile, 15-35wt% of cellulose acetate and a solvent, and carrying out heating, cooling and filtering to obtain a spinning stock solution; wherein a ratio of the sum of the masses of the powdery polyacrylonitrile and cellulose acetate to the mass of the solvent is (20-25.5):(74.5-80), the solvent is select from a group consisting of dimethylacetamide, dimethyl sulfoxide, dimethylformamide or sodium thiocyanate; and 3) subjecting the spinning stock solution to temperature adjustment and pressure adjustment, carrying out spinning at a temperature of 80 DEG C to 97 DEG C and under a pressure of 0.7MPa to 0.9MPa after filtering, carrying out double-diffusion shaping in a coagulating bath, water washing, drafting, oiling, baking, curling and setting, thereby obtaining the fiber of polyacrylonitrile -cellulose acetate, wherein, the coagulating bath is selected from an aqueous solution of dimethylacetamide, dimethyl sulfoxide, dimethylformamide or sodium thiocyanate and has a concentration of 30-55wt% and a temperature of 25 DEG C to 50 DEG C.