This paper is based on the framework of cognitive flexibility theory, this study intends to test the vocabulary skills of three groups of subjects representing low, middle and high levels of English and explored the following three main questions:1. Whether there are differences in second-language vocabulary competence in the classroom environment for subjects with different language levels. 2.whether there are differences in the cognitive flexibility of subjects with different language levels. 3. whether there is a correlation between cognitive flexibility and classroom second language vocabulary competence. The purpose of this paper is to provide experiences for developing learners' ability to apply their knowledge flexibly.