Disclosed is a method for preparing bioethanol from biomass. The feedstock for preparation of bioethanol, i.e. the biomass, is pretreated by a combination of chemical and physical methods. The method is advantageous in that detoxification is unnecessary since the substances that inhibit fermentation are not produced and acid reconcentration for recycling is not needed. Further, since the sonication makes saccharification easier, removal of lignin (delignification), which interferes with the saccharification of cellulose, is unnecessary. Accordingly, the present disclosure allows to produce bioethanol with high yield and at low cost in an environment-friendly manner.