In this paper，in order to obtain more robust digital watermark scheme，firstly the theory of iterated function system（IFS）and the stochastic algorithm by which IFS attractor is constructed are proposed Secondly，by analyzing the robust character against geometric distortion of IFS attractor theoretically，a new scheme is provided，which is a asymmetric digital watermark one Thirdly，using tri point method，the paper translates the information of digital watermarking into IFS code，and the indices set can be attained by reconstructing the image of fractal watermarking and manipulating the original image relatively Finally the index set is signed and time stamped by the owner and trusted third partys private keys Using the owner and trusted third partys private keys，the watermark can be detected without the original image The experimental results show that the watermark scheme is robust against image manipulations，such as noise adding，filtering，compression and rotation