Bitmap-based collaborative graphics editing systems are a special class of real-time collaborative editing systems. Undo is an important and difficult problem in these systems. Existing solutions show low efficiency because additional space cost should be added to achieve the function of undo. In this paper, we propose a new solution to resolve the undo problem. The basic idea is to reduce space cost through exploring relations among operations. The algorithm given in the paper can undo any operation a any time, and can greatly reduce the space cost while increasing a few time cost. The issues about undo scope and undo mode are also discussed in the paper.