Abstract: Two kinds of self-breakdown water switches with different configurations for Qiangguang-I accelerator were described. The electrical model of the water switch was built. By estimation and Pspice calculation, the parameters of the switches were determined. The acquired breakdown characteristics of the two different switches by experiments were described by two different breakdown models. The critical field of the transfer switch could be calculated by J. C. Martin empirical expression about water breakdown with a mostly uniform field. The critical field of the output switch could be calculated by J. C. Martin expression about divergent field of point-plane electrodes. The experimental data of breakdown of water switch show that the above-mentioned methods for breakdown field calculation are