Cloud computing is regarded as one of the most enticing technologies and potential silver bullet in the IT industry. Because of open condition and general-purpose nature of cloud, security issue is becoming a bottle neck of rapid development and broad application of cloud computing. Firstly, this paper addresses cloud customers’ significant concerns about and requirements of cloud security. Secondly, cloud security risks and threats posed by the pervasive and ad hoc nature of the cloud are comprehensively analyzed. Finally, some effective and dependable security mechanisms are proposed to lower cloud security risks and meet security requirements of cloud customers. Especially, a novel scheme for integrity and copyright protection of customers’ works in the cloud is presented based on digital watermarking and digital signature.