Abstract: To obtain the deniable property, Lee and Chang proposed a strong designated verifier signature scheme without hash functions, and also constructed a strong designated verifier ring signature scheme.In this paper, first, it is pointed out that forgery still exists for the strong designated verifier signature scheme, then, although the strong designated verifier ring signature scheme is safe, yet it is inefficient.Last, an improved strong designated verifier ring signature scheme is proposed with the safety conditions unchanged.Cases study show that counterfeit is prevented by using the improved scheme and more efficiency is attained.