The invention discloses a biological modification method of lactic acid fibers with good spinnabillity. The method comprises the following steps: charging various raw materials in to a high-speed mixer, and dispersing the raw materials at a high speed to obtain a modified polylactic acid mixture; melt blending and extruding the uniformly-mixed modified polylactic acid mixture by virtue of a dual-screw extruder to obtain modified polylactic acid master batch, and vacuum drying; placing the modified polylactic acid master batch and vacuum dried ordinary polylactic acid slices into a high-speed mixer, uniformly mixing, spinning the mixture on a melt spinning machine by virtue of a circular spinning hole to obtain the modified polylactic acid fibers, then carrying out the microorganism modification, and strictly controlling fermentation parameters. Different from the ordinary constant-temperature fermentation, by adopting the instantaneous heat treatment, a purpose of shortening the fermentation time and obtaining an unexpected modification effect can be achieved.