The fifth generation mobile communication network, abbreviated 5G network or 5G, is also called the next generation of mobile communication network, which aims at constructing a networked society and realizing the goal of "everything-connecting". With 4G mobile communication network entering the commercial stage, the research on 5G has gained wider attention all over the world. The realization of the vision for 5G needs the revolution and innovation of system structure and core techniques, and until now the corresponding techniques and standards are in the primary stage. The new architectures, new business, and new technology bring new challenges to 5G security. This paper briefly summarizes performance, key technology, application scenario, and standardization progress of 5G, analyzes the security requirements and challenges of 5G, introduces 5G security framework, and investigates some key issues and the corresponding solutions based on the research efforts, white paper and related standards. Furthermore, the paper discusses the current research trends in industries and academia in the context of 5G security.