1. A speech synthesis method comprising: storing a group of speech units in a memory; segmenting a phoneme string of a target speech, to obtain a plurality of segments; selecting, from the group in the memory, a speech unit for each of the segments based on prosodic information of the target speech, to obtain an optimal speech unit sequence including speech units selected for the respective segments; selecting M (M represents a positive integer greater than one) speech units for each of the segments from the group in the memory, based on the optimal speech unit sequence; generating a new speech unit corresponding to each of the segments, by fusing the M speech units selected for said each of the segments, to obtain a plurality of new speech units corresponding to the segments respectively; and generating synthetic speech by concatenating the new speech units.