The aim of this paper is to investigate the separation of oral and written language in ancient Chinese prose of Northern Song Dynasty. We all know that the classical Qin-Han Archaic Chinese has always been the writing model of the ancient prose movement, while on the other hand during Northern Song Dynasty the oral language (so called BaiHua) has already developed quite differently from the written language (so called WenYan). Thus, it becomes very interesting to ask the question that in spite of the great language change, did those famous prose masters in Northern Song Dynasty all write in strict classical written style without any influence from the contemporary oral language? In this paper, we choose SuShi, the most distinguished prose writer of Song dynasty, to be the study case since he wrote a lot of great works of ancient prose as well as Song Ci. The syntactic analysis of SuShi’s works reveals that, firstly, SuShi’s writings did exhibit the typical case of separation of oral and written language. His prose strictly followed the syntax of Archaic Chinese, while his ordinary letters and “Ci” showed many features from the oral BaiHua language of the Northern Song Dynasty. Secondly, SuShi’s prose followed the syntactic rules of the Archaic Chinese, such as the object pronoun fronting in negative and interrogative sentences. On the other hand, his daily letters and Ci used the “V+ de” construction and many other contemporary oral words such as “mou”, “qu”, “ge”, “sheme”, “zi” etc. Thirdly, although utilizing some oral style in his daily writing, SuShi’s attitude toward writing was as traditional as other ancient prose writers; they all worshiped the classical written form of the Archaic Chinese and considered the oral language style to be vulgar and informal.