Being a feature of language, sentence-length, which is objective and can be easily manipulated, is the externalization of texts-internal characteristics. The research attempts to review the studies that focus on sentence-length in written language and to predict the prospect of future research, so as to enlighten L2 teaching and learning. Based on research focus, the existing studies are chronologically reviewed. Results show that the existing studies mainly concentrate on style, text classification, language properties and readability. While laying a sound foundation for future research, the existing studies show certain limitations, namely, the monotony of research model and the exclusion of the written language related to L2 learners. Predictions are made that future research focuses more on the sentence-length in written language related to L2 learners. Meanwhile, observing the principles of triangulation, future researchers from different disciplines should cooperate to reveal the truth of sentence-length of written language related to L2 learners by triangulating data, research methods and theories.