The application of phonology to the theory of the Kunqiang duqu of traditional Chinese opera, which occurred in the Ming dynasty, is best represented by the works of Wei Liangfu (1526-1586) and Shen Chongsui (?-1645), two dramatists separated by a century. Their works also mark the climax in the innovations and evolution of the genre of kunqu. Their works deal with three aspects of phonology important to the theory of kunqiang duqu. First, Wei Liangfu brought up the simple principle that, "There are two situations that do not allow mixtures of qu. One is that the Southern songs or qu cannot be mixed with the Northern singing style or qiang; and the other is that the Northern qu cannot be combined with the Southern words." Shen Chongsui took this principle as a central issue, and furthermore established the theory: “Northern phonology is consonant with rhymes in the rime book Zhongyuan yinyun; while the South follows the authentic rhyming of the Hongwu reign period (the rime book titled Hongwu zhengyun)." Second, Wei Liangfu proposed that singers should increasingly modify dialect languages and local accents. Shen Chongsui took up this point, urging that central accents not be muddled up by local ones, and introduced the theory of "modeling sounds on the central standard and thus correcting the errors of the Southern sounds." He used six standard notations to structure the theory. Third, Wei Liangfu broached the principle of "examining even, ascending, falling, and entering tones (ping, shang, qu, and ru) individually and being sure that the four intonations each reach their standard." Moreover, Shen Chongsui was unsurpassed in formulating the essence of singing the four intonations; he founded their main principles. The paper will discuss the evolution of the theory of Kunqiang duqu and observe the heritage and development from these three threads of thought.