Abstract: An improved algorithm of hollowing STL model is presented which takes advantages of voxel method and offsetting method. The voxel method can hollow out any arbitrary complex STL model, and the offsetting method has very simple calculation. This algorithm can improve the voxel representation, accelerate the speed of labeling different type of voxels, and smooth the hollow slices. Consequently, the algorithm is very stable, robust and fast. In fact it has been applied to rapid prototyping domain in success, and as a result greatly improved the RP material consumption and building time required.