The invention discloses a method for preparing polyacrylonitrile carbon fiber protofilament by dry and wet methods. The method comprises the steps of polymerization, demonomerization and defoaming, filtration, coagulation, washing and drafting, oiling densification, steam drafting, heat setting and drying. Three-level coagulating baths at the temperature of between 10 DEG C below zero and 70 DEG C and with dimethyl sulfoxide with concentration of 10 to 60 mass percent are adopted in the coagulation step, and the first coagulating bath contains aqueous ammonia accounting for 0.05 to 1 percent of the mass of the first coagulating bath; and a spinning head is subjected to 1.5 to 5 times positive drafting in the first coagulating bath, and the drafting is 0 in the second and third coagulatingbathes. According to the method for preparing the polyacrylonitrile carbon fiber protofilament, the spinning process is stable, the broken filament is little, the spinning speed is high, the spinningis stable, the prepared protofilament has few defects, the density is not less than 1.180g/cm<3>, and the tensile strength is not less than 7cN/dtex. The protofilament can be prepared into a high-performance carbon fiber with tensile strength of more than 4.9GPa and elastic modulus of between 260 and 280GPa by high-temperature carbonization.