Abstract: The case study is one of the most common methods in Chinese as a second language study. First, the paper, with visualization analysis, reviews the developmental trend, hot topics, and useful tools of the case studies in the field of Chinese as a second language. Besides, this paper holds that the terms "个案研究" and "案例研究" (both refer to the case study in English) are mainly consistent. So that case study("个案研究") can be used as a unified term for this research method. Finally, this paper proposes that the case study could be one method in all aspects of research on Chinese as a second language. Meanwhile, four features of a high-quality case study are proposed.