The invention relates to a method for preparing carbon nanofibers from wearing acrylic fibers or waste wearing acrylic fibers, and belongs to the technical field of preparation of the carbon nanofibers. The method includes performing washing, impurity removal and drying pretreatment on the wearing acrylic fibers or the waste wearing acrylic fibers, then re-dissolving the acrylic fibers by good solvents, adding metal salt for modifying, controlling modification temperature and time to obtain spinning precursors, and performing spinning, pre-oxidation and carbonation on the spinning precursors to obtain the carbon nanofibers. The metal salt for modification refers to one or two or more of cobalt acetate, manganese acetate, zinc acetate, ferric acetate, nickelous acetate, copper acetate, magnesium acetate, sodium acetate, cobalt chloride, zinc chloride, manganese chloride, ferric chloride, copper chloride, cuprous chloride or stannous chloride. Raw materials applied to the method are waste. The method has the advantages that cyclization reaction triggering mechanisms of the acrylic fibers during pre-oxidation can be changed, cyclization reaction starting temperatures of the acrylic fibers can be lowered remarkably at the same time, energy conservation and environmental protection can be achieved, and production costs can be reduced remarkably.