The present invention relates to a method for preparing a vinyl chloride-based polymer, more specifically, to a method for preparing a vinyl chloride-based polymer through which it is possible to improve an apparent specific gravity of a vinyl chloride-based polymer to be prepared by controlling a surface tension of a polymerization mixture to 46.5 mN/m or more, and thus, to improve an extrusion amount when extruded, whereby a vinyl chloride-based polymer having excellent processing productivity can be prepared.