In this paper we present some numerical analysis for wave-head of pulse voltage on a water switch which is used in the High Power Pulse Water Dielectric Switch Research Device，i.e. No. 2 device in Lab. 206. We have theoretically discussed a variety of parameters which can affect characteristics of the water switch. We have drawn an important conclusion that C_M/C can evidently affect some characteristics of the Water switch. Thereby，a theoretic evidence is suggested for the design of a high power submicrosecond water switch. A lot of experiments' data in our device have confirmed our numerical analysis.