Electrohydraulic shock wave (EHSW) is seemingly one of the simplest and most common products of microsecond pulsed discharge (μsPD) in water; however, its generation process remains far less clear. To study the influence of current waveforms on the generation of an EHSW, we conducted discharge current interception experiments using a bypass branch in the circuit. The current interception time Δt is properly controlled so that the discharge current through the water gap can be terminated at a chosen time. Results show that the peak pressure Pm is first linearly increasing with Δt, and then Pm reaches a stable value. The expansion of the spark channel with increasing velocities will enhance the peak pressure. This phase can be regarded as the accelerated expansion phase (AEP) of the piston theory. The transition area of the Pm-Δt relationship of this experimental setup shows that the AEP lasts for about tm = 5 μs. After the AEP, the deposited energy will help to maintain a higher pressure in the falling edge of the pressure waves. The full width at half maximum of the waveforms finally approaches 12.5 μs in our tests. The experimental results provide evidence of the piston theory in interpreting the generation of EHSW induced by μsPD.