Electrohydraulic discharge is an efficient and economical method for generating repeated shock waves (SWs) without load feeding devices. However, low energy conversion efficiency of electrohydraulic discharge (< 5%) limits the shock wave intensity generated in the narrow space, such as down wells, mines, etc. This paper proposed a gel-like nitromethane-based energetic materials (EMs) and its feeding device, which can be integrated into shock wave devices based on electrohydraulic discharge. The feeding device can fill gel-like EMs into the water gap and maintain shape for more than 5 min without the shell. Then medium energy storage (~1 kJ) pulsed discharges can generate arcs to ignite EMs: with 2.4 g EMs, the SW amplitude, impulse and energy are respectively increased by 1.8, 7.6, and 31.6 times compared to electrohydraulic discharge. The mechanism of arc ignited EMs is analyzed by combining optical diagnosis and electrical measurement. It is found that high electric field strength triggers local breakdown and initial reaction of EMs. As the reaction zone progresses towards both ends, an arc channel is formed to bridge the electrodes, allowing the continued deposition of electrical energy to ignite EMs and maintain the development of detonation wave. Four discharge modes are divided according to nitromethane volume fraction. Among them, matching discharge mode generates the highest SW amplitude and breakdown discharge mode has the highest ignition reliability.