Micro-EDM milling is an effective machining process for three-dimension micro-cavity of high hardness materials. Because of the sharp wear of the electrode, fix-length compensation method is proposed, thus results in a coneshaped electrode. In order to give a clear explanation of the forming mechanism, this paper models the electrode and workpiece geometrically and mathematically as two-dimension matrices and simulates the shape change during machining. An experiment was conducted to verify the simulation. The results show that this simulation predicts the profile of the electrode and the workpiece very well in the fix-length compensation process.