In this paper, we present an improved interactive watershed algorithm for bone removal from CTA (Computed Tomography Angiography). The improved watershed algorithm builds upon the sorting step of the fast immersion-based watershed transform. After the sorting step, each element is processed with the respect to its neighborhood in the specified order, and the resulting basins are organized in a tree structure. After that, the tree partitioning and basin merging are controlled by one global parameter called merging value and some markers are given by users to suppresses the over segmentation. The experiment indicates that the algorithm prevents the over segmentation effectively, and removes the bone structure excellently.