In this paper, the issues of entity authentication and content authentication mechanism of the digital signature based on multimedia system are addressed. Based on the analysis of the security gap in the authentication mechanism using digital signature, a novel authentication protocol combining digital signature and fragile watermarking are proposed. Double entity authentication based on digital signature and content authentication base on fragile watermarking is achieved. Illegal access to system, edition, and forgery of multimedia document is proven to be impossible. According to the analysis, the security, creditability, and authenticity are achieved.