Electrochemical machining (ECM) is an important non-conventional manufacture technology for machining workpieces with complex and thin structures. In this study, ECM is used to machine the blisk. Because the channel between two blades is narrow, the cathode must use the thin-sheet structure. In the ECM process, the cathode will bear electrolyte pressure which can cause deformation. The cathode deformation has negative influence on the machining quality and process stability. To minimize the deformation, the optimization treatment of the cathode thickness is carried out and a new electrolyte flow mode, W-shaped flow mode, is adopted to weaken the pressure. The corresponding computer simulation is applied, and the results show that the cathode deformation decreases by 17.6% and 28% by taking the above-mentioned two measures. It indicates that the new thin-sheet cathode presented in this paper is useful and the new flow mode is effective in ECM. Furthermore, the experiments are conducted using the optimal thickness electrode and W-shaped flow mode to verify the machining effects. Then the corresponding experiment is carried out, and the result shows that the optimal treatments to reduce the cathode deformation have remarkable effects.