This paper mainly analyzes the theory researches about the complex sentence in the first half of the 20th century from the perspective of classical works. In the aspect of distinction between simple sentences and complex ones, there have five standards, that is, “simple sentence”, “subject predicate phrases” and “sentential form”, “structural relationship”, and “speech pause” and “multi-combinations”. In the aspect of the classification of complex sentences, there have been three classifications, including “single-way split”, “dichotomy”, and “three-way split”.