The method includes following steps: when picking up parameters of frequency spectrum, the method considers difference between orders in line spectrum frequency as a part of picked up result; when modeling a model and training, carrying out independent modeling and training for line spectrum frequency and parameters of difference between orders; when making prediction, predicting line spectrum frequency and parameters of difference between orders respectively, and moreover carrying out adjustment for parameters of frequency spectrum by using difference between orders; finally, using adjusted parameters of frequency spectrum synthesizes output voice in order to reach purpose of raising tone quality of synthesized voice through enhancing and sharpening formant of synthesized voice.