Abstract: In order to obtain multi-boundary of digital image quickly, a new boundary tracking algorithm based on searching direction estimation is proposed. The model of multi-boundary and the initial feature point method were studied. Four direction operation templates and a searching direction matrix were defined to estimate searching direction of next step. For enhancing the robustness of tracking algorithm, a weight matrix was defined to keep tracking algorithm out of searching trap. Tests were performed on MR images, and the results show that the tracking efficiency of new algorithm is enhanced by 21% compared to variable window reptile method or traditional tracking algorithm. According to the test results, the robustness of the new tracking algorithm is improved obviously.