Code-switching (CS) has received much attention from researchers . However, the great amount of studies on CS done so far are mostly focused on spoken CS rather than written (or typed) CS . Even though such studies have been carried out from different approaches especially in respect to linguistic features and socio-linguistic factors, CS in computer-mediated communication in China has been rarely touched upon. This research studies written or typed CS in a recently popular form of asynchronous CMC in Chinese Mainland—Weibo, or "Microblogging". With data from entries of twelve subjects from Sina Weibo, the newest Microblogging web in mainland China, as well as entries from Leiden Weibo Corpus, the current study explore the major types, functions , and possible motivations of CS in Weibo. Six types of CS in this corpus are sorted out, which are inter-sentential CS, intra-sentential CS, intra-word CS, tag-switching, inter-turn CS and intra-turn CS. And there are eight major functions of CS in Chinese Microblogging including quotations, specificity, lexical gap filling, euphemism, qualifying message, expedient, language mixing, injecting humor, medium repair and other-language repair. Possible factors that motivate the use of CS in Chinese Weibo are primarily social identity construction and negotiation, personal motivation—better expression of emotion and/or message, characteristics of Weibo itself—the language variety of Weibo and its users' creativity. Topics for further studies are also suggested.