A blind image watermarking scheme based on secret sharing in discrete wavelet transform domain is proposed. Watermark was divided into n shadows according to secret sharing scheme. And t or more of those shadows can reconstruct the watermark, while t-1 or less shadows could not do it. In order to achieve optimum embedding strategy, a closed loop embedding process is proposed, which is modified iteratively according to results of performance analysis. The convergence of closed loop watermarking is proved. Independent component analysis is utilized so that detector can not merely detect watermark but also can extract it. Before watermark reconstruction, one way hashing function is used to withstand cheating attacks. The experimental results show that it is robust against a wide range of attacks proposed by Stirmark and it is more safety than traditional watermarking techniques.