Abstract: A method is proposed to get the bottom line of intonation phrase by the bottom points and sub-bottom points of tone range in Standard Chinese. And on the basis of the bottom line, the intonation features of the natural sentences drawn randomly from the large-scale speech database were explored. The primary research results demonstrate that different from the view that the intonation in Chinese only appears at the ending of sentence, there exist in Standard Chinese the intonation features of the pitch declination and resetting universally found in other languages. The declination and resetting can be observed in different prosodic units such as foot, intonation phrase and prosodic utterance, and most obviously in intonation phrase. The research result supports the view that the top lines and the bottom lines have different linguistic functions in the two-line model of Standard Chinese intonation.