1. A parametric speech synthesis method, comprising: a synthesizing phase in which each frame of speech of each phone in a phone sequence of an input text is sequentially processed as follows: for a current phone in the phone sequence of the input text, extracting a corresponding statistic model from a statistic model library and using model parameters of the statistic model that correspond to the current frame of the current phone as rough values of currently predicted speech parameters; according to the rough values and information about a predetermined number of speech frames occurring before the current time point, filtering the rough values to obtain smoothed values of the currently predicted speech parameters; according to global mean values and global standard deviation ratios of the speech parameters obtained through statistics, performing global optimization on the smoothed values of the currently predicted speech parameters to generate necessary speech parameters; and synthesizing the generated speech parameters to obtain a frame of speech synthesized for the current frame of the current phone.