In order to improve the compact design level and insulation reliability of the flexible DC converter valve equipment, this paper focuses on the compact design optimization problem of the air clearance between the layers of the flexible DC converter valve. Combined with the test under different conditions and the corresponding electric field simulation analysis, the discharge characteristics of the typical gap between the valve layers are studied, and the optimization method of the air clearance design between the valve layers is proposed. The test part of this study used switching impulse test, AC breakdown test and DC breakdown test to test the breakdown discharge voltage corresponding to the electrode spacing between valve layers under different conditions, and obtained the fitting curve of discharge characteristics. The simulation part of this study uses the COMSOL electrostatic field steady-state module to calculate and analyze the electric field distribution and the maximum surface field strength of the typical gap between the valve layers. After comparing the test and simulation results, the breakdown discharge position is consistent with the maximum position of the surface field strength, so as to obtain the reference value of the breakdown field strength corresponding to different voltage waveforms. Finally, the air clearance is optimized by using the discharge characteristic curve of the typical gap between the valve layers. This study provides a theoretical basis and engineering guidance for the compact design of the flexible DC converter valve.