The invention relates to a preparation method of a polylactic acid side-by-side composite fiber. Polylactic acid slices and modified component slices are subjected to three-stage drying and screw melting respectively and then enters a spinning box for side-by-side compounding at a spinneret plate, the side-by-side composite fiber out from the spinneret plate is subjected to air blast cooling, oiling, winding and traction, and silk accommodating sequentially, and after oil immersion water-bath stretching, steam stretching, oiling and cooling, and heat setting, the polylactic acid side-by-sidecomposite fiber is obtained; a modified component is selected from one or combination of dacron and chinlon; the mass ratio of polylactic acid to the modified component is 80:20 to 50:50. The polylactic acid side-by-side composite fiber is good in crimp tendency and temperature resistance, and suitable for being applied to filling materials or clothing fabrics.