1. A parametric speech synthesis method, comprising: analyzing an input text; acquiring a phone sequence based on analysis of the input text, the phone sequence including a plurality of speech frames; synthesizing the phone sequence by synthesizing the plurality of speech frames in a sequential manner, each speech frame being synthesized by performing the following iteration; extracting a corresponding statistic model from a statistic model library and using model parameters of the statistic model that correspond to the speech frame as rough values for predicting speech parameters of the speech frame; according to the rough values and information about a predetermined number of preceding speech frames, filtering the rough values to obtain smoothed values for predicting speech parameters of the speech frame; according to global mean values and global standard deviation ratios of speech parameters obtained through statistics, performing global optimization on the smoothed values to generate speech parameters of the speech frame, wherein the global optimization comprises the global mean values and global standard deviation ratios being fixed values using the same values for adjustment in each speech synthesis process without the need of recalculating the global mean and the standard deviation ratios in each speech synthesis process; and synthesizing the optimized speech parameters to obtain a frame of speech waveform.