1. A method of speech segment selection for concatenative synthesis based on prosody-aligned distance measure, comprising the steps of: (A) segmenting speech stored in a speech corpus into at least one speech segment according to a unit type, wherein each speech segment has its prosody information; (B) locating pitch marks for each speech segment; (C) selecting one of the speech segment according to the unit type as a source segment and other speech segments as target segments, and performing a prosody alignment between the source segment and each target segment to obtain a prosody-aliened source segment, wherein the pitch marks of the prosody-aligned source segment are aligned with the pitch marks of the target segment; (D) measuring distortion between the prosody-aligned source segment and each target segment to obtain a distance between the prosody-aligned source segment and each target segment, and to obtain an average distance between the prosody-aligned source segment and each target segment; and (E) selecting at least one speech segment with a relative small average distance.