The invention relates to a method for preparing a carbon fiber precursor lignin/polylactic acid fiber. The method comprises the following steps: carrying out intermittent heat treatment on purified lignin powder and polylactic acid slices in a vacuum drying oven; mixing the lignin powder subjected to heat treatment with the polylactic acid slices; pelleting at 160-250 DEG C through double screw rods to obtain lignin/polylactic acid composite master batches; and spinning the dried lignin/polylactic acid composite master batches at 190-250 DEG C through a melt-spinning machine, so as to obtain the carbon fiber precursor lignin/polylactic acid fiber. The carbon fiber precursor lignin/polylactic acid fiber provided by the invention has the advantages of renewable raw materials, excellent fiber quality, small diameter of obtained fiber, high strength and low cost; continuous production can be carried out; the carbon fiber precursor lignin/polylactic acid fiber is expected to be applied to the fields such as automobiles, high-speed rails, buildings and sports goods as a reinforcing material after being carbonized; and the market prospect is wide.