In order to solve the issue of the poor rigidity of cathode structure in the electrochemical machining (ECM) of titanium alloy with high length-width ratio, a hollow slice cathode with stiffeners was designed. The effects of the number of stiffeners and the angle between stiffener and sidewall on the overall rigidity of the cathode were studied by one-way fluid coupling analysis method. The simulation results show that the overall rigidity of the cathode is improved remarkably when three stiffeners are added to the cathode structure. In addition, the overall rigidity of the cathode decreases with the increase of the angle between stiffener and sidewall. Comprehensive cathode deformation and the maximum equivalent stress, the angle between stiffener and sidewall is 45°, the cathode structure has good rigidity, and can meet the rigid requirements. Finally, adopt self-developed experimental system to implement a ECM process test of deep narrow grooves with high length-width ratio. The optimized cathode structure can be used to process a deep narrow groove with lengthwidth ratio of 11 ∶ 1 and depth-width ratio of 9 ∶ 1.