Digital watermarking technology plays an important role in the areas of copyright protection and identity tracing for owners of digital mediums. At present, the security of the watermarking scheme is facing a great threat. The security of a digital watermarking scheme must not depend on the scheme being kept secret. Zero knowledge-based watermark detection scheme (ZKWD) can achieve this aim. For ZKWD scheme, an owner can provide prove to a verifier that a digital medium in question indeed contains the owner’s watermark information without revealing any secret key and watermark-related information. However, the existing ZKWD protocols are still facing some challenging problems, such as ambiguity attacks. In this paper, a public ZKWD protocol is proposed for plain text, and the homomorphic property of asymmetric encryption algorithm in the multiplication operation is used to prevent the owner from cheating by ambiguity attacks. Compared with existing methods, the security of our proposed ZKWD scheme is improved by using the improved feature extraction algorithm.