Abstract With great developments in society and technology, the phenomena of inter-cultural communication, bilingualism and multilingualism have become more and more common. People tend to choose a proper language when communicating with others, consciously or unconsciously. Language choice may vary according to different environments, participants and other social or discourse factors. When language choice changes, speakers would engage in code-switching and code-mixing. This study focuses on four bilingual junior high students aged from 1 1 to 13 in a junior high school in Shijiazhuang, Hebei Province. They have received years of English education, and Mandarin Chinese is their first language. Adopting sociolinguistic approach, the research aims to find out the answers to the following research questions: Under bilingual education and in after-class discussions, (1)what are the types of code-switching of these Chinese-English bilingual junior high students aged from 11 to 13 in the northern part of China? (2)What are the functions of their code-switching and why did they code-switch in different circumstances in after-class discussions? Junior high students, as a social group of language learners, same as adults as well, choose the proper language in different contexts. The major findings of this research are: although the informants produce all kinds of code-switching, inter-sentential code-switching occur most frequently; and the main functions of their code-switching are quotation, correction, reiteration, encoding authority, changing topics, discourse markers and word play.