Linguistic conditions and the refinement of language style are the prerequisite and facilitator for the popularity of specific poetry styles in Chinese classical poetry. The formation of four-character sentence was the linguistic prerequisite for seven-character poetry, and was also the fundamental reason for the decline of four-character poetry relying on “two-two” rhythm. The improvement of seven-character style resulted from imperial order was an important impetus for the rapid development of seven-character poetry. The grammatical reason behind the parallel between seven-character poetry and five-character poetry highlighted their difference, making seven-character lines more suitable to write about scenery, emotion and argument because of its representation space and grammatical density.  In terms of prosody, the “four-three” structure of seven-character line formed an expressive feeling that was more decorative and explicit than five-character sentence.