The present invention relates to a method for preparing an extract containing omega fatty acids from a plant containing omega fatty acids, including the following steps: drying the plant containing omega fatty acids to prepare a powder; and extracting omega fatty acids from the prepared plant powder by using supercritical carbon dioxide as a solvent at an optimum temperature and pressure, a dietary supplement and a cholesterol-lowering pharmaceutical composition containing plant-derived omega fatty acids extracted by the method, and omega fatty acids-containing plant extract containing a predetermined or greater amount of omega-3 fatty acids.