A method for performing speech synthesis on textual content at a client. The method includes the steps of: performing speech synthesis on the textual content based on a current acoustical unit set Scurrent in a corpus at the client; analyzing the textual content and generating a list of target units with corresponding context features, selecting multiple acoustical unit candidates for each target unit according to the context features based on an acoustical unit set Stotal that is more plentiful than the current acoustical unit set Scurrent in the corpus at the client, and determining acoustical units suitable for speech synthesis for the textual content according to the multiple unit candidates; and updating the current acoustical unit set Scurrent in the corpus at the client based on the determined acoustical units.