In outsourced database system, data owners delegate their database needs and functionalities to a third-party that provides services to the users of the database. Since the third party can be seldom trusted or can be compromised, it is important to provide copyright protection for outsourced database. In this paper a novel scheme of copyright protection for outsourced database based on digital watermarking is proposed. In the presented system, watermark is embedded into the database under the control of both the secret key and the chaotic random numbers. No original database is needed during watermark detection, and the copyright is judged by the match rate of watermark. The analyses of attacks towards watermark and the results of the experiments show that this system possesses high robustness and availability.