This paper proposes an independent system of prosodic application used in ”Lisao” 離騷(attr. Qu Yuan, c.340-278 B.C.), different from the one in ”Shijing” 詩經 (Book of Odes). It is shown that while a fixed dipodic prosody (二步律) was used in ”Shijing”, a caesura-based prosody (頓歎律) was developed in ”Lisao” during and after the Warring States period (475-221 B.C.). By the Han dynasty (206 B.C.-220 A.D.) however, flexible caesura prosody was gradually replaced by syllabically-fixed patterns: first trisyllabic, then pentasyllabic and finally heptasyllabic. To date, there has been no reasonable explanation proposed for why and how only odd numbers of syllables in lines developed. This paper deals with this question from a prosodic point of view. It is argued here that poetic evolutions in classical Chinese are determined by the grammar of poetic prosody.