Shock wave focusing can produce higher pressure in the focal region. An underwater shock wave focusing system was set up based on the pulsed discharge in water and the focusing characteristics of ellipsoidal reflector. The focusing of shock wave was studied experimentally; the focusing process of shock wave was analyzed. The pressure-time history and the peak pressure along the axial position were presented, the distributing characteristics of the focusing shock wave were revealed. Negative pressure and cavitation phenomenon was induced by shock wave focusing, and the phenomenon was analyzed based on shock dynamic theory. The experimental results indicate that the focusing effect of the ellipsoidal reflector is remarkable.