Abstract: The static characteristics of 3 MV multi-stage multi-channel closing switch(MMCS) should be defined. The self-breakdown characteristics of the MMCS with different number of gap stages and the trigger gap were studied by reducing the number of electric gap stages. The results indicate that the self-breakdown voltage varies nonlinearly with the gap distance， which deviates from the theoretical value obtained by Bradley empirical formula. The static electric field strengths of multiple gaps in series were calculated by using Ansoft program. By analyzing the experimental data， electric field uniformity coefficient and Bradley empirical formula， a new Bradley extrapolation formula was obtained， which can better reflect the self-breakdown voltage of MMCS and reduce the discrepancy between experiment and theory. The relative error was reduced to 5% by using the Bradley extrapolation formula.