Abstract: With the development of ubiquitous computing technology and ubiquitous operating system (UOS), ubiquitous computing has become a hot research topic both in industry and academy. As classic ubiquitous computing scenarios, smart home, industrial Internet of things, self-driving, and cloud computing, have become increasingly prosperous, and their security issues have attracted the attention of researchers. Currently, as the related research on ubiquitous computing security is in its initial stage, there is still no general security methods that can solve the emerging security issues of ubiquitous computing. In this paper, we firstly review the current status of ubiquitous computing, UOS, and summarize its architecture. Then, we analyze and summarize the state-of-the-art research effort on ubiquitous computing security, and divide security issues into three major aspects: system security, device security and communication security. We discuss the security issues and related research effort in four classic ubiquitous computing scenarios. Through in-depth analysis of the shortcomings of existing research and the causes of security problem, we summarize eight key technical challenges and opportunities in ubiquitous computing security. Finally, we discuss every challenge, and point out the potential security research directions of ubiquitous computing in future.