© 1994-2012 IEEE. Shock waves generated by underwater pulsed discharge have been applied to industrial and bio-medical treatments, and the key technology is to induce high intensity shock waves. Based on the underwater pulsed discharge platform, the effect of the electrical breakdown modes on the shock wave intensity in water is discussed in the positive pin-to-plane configuration. Experimental results demonstrate that the amplitude of applied voltage have a significant influence on the breakdown modes as well as the shock wave intensity. Based on the analysis of discharge images captured by a high-speed camera, it can be concluded that the breakdown modes are classified as subsonic bush-like streamers or supersonic filamentary streamers. The mechanism of subsonic streamer in lower applied voltage is more likely an electro thermal process. The micro-bubbles are firstly generated, and then discharge occurs in the bubble cluster. The bubble volume grows until a breakdown of the underwater gap, and the shock wave is the generated. The time to breakdown of subsonic streamers is commonly within hundreds of microseconds at the experimental condition. In higher applied voltage, the energy loss in the pre-breakdown process is reduced significantly as well. In our experiment, a threshold value of 22.5 kV is necessary for the subsonic streamer mode transition into supersonic streamer mode. The energy conversion efficiency, the time to breakdown and the influence of the applied voltage are discussed at different breakdown modes. The shock wave intensity differs 2-4 times in different breakdown modes even in the same applied voltage, thus increasing the applied voltage may be a remarkable way to enhance the shock wave intensity.