The word de 得is generally used to express capability in the affirmative forms of Chinese potential structure. But the negative potential structure V-bu 不-C without de can also express the meaning of capability. What the origin of potential interpretation in V-bu-C is remains to be a mystery in the research of Chinese syntax. This paper proposes that there is a syntactic position in V-bu-C expressing the potential meaning. However, the reason de does not appear in the phonological form depends on prosody. Specifically, de cannot appear in the syntactic environment with both object and complement. This can be attributed to the mechanism of the nuclear stress assignment in Mandarin. Being the nuclear stress assigner, V-bu-C must be in conformity with the minimal word condition (not greater than or equal to three syllables), as a result of which de cannot show up in the phonological form. Such an analysis not only answers the question of what the source of the potential interpretation of V-bu-C is from the view of syntax, but also deduces and interprets various forms of potential structures.