Experiments were conducted in a Mach 2.5 wind tunnel to explore the modification effect on the shock wave structure by a plasma spike generated by an on-board 60 Hz electric discharge in front of a 60° cone-shaped model, which was used as a shock wave generator. Modification of the shock by the plasma spike from an attached conic shock into a highly curved shock structure, which has increased shock angle and also appears in diffused form, is demonstrated. Due to the cyclic nature of the generated plasma an unsteady shock motion during one discharge period was observed. As shown in a sequence with increasing discharge intensity, the shock in front of the model moves upstream to become detached with increasing standoff distance from the model and with increasing dispersion in the spatial distribution of the shock front. Experimental results exclude the heating effect as a possible cause of the observed shock wave modification. A theory also using a cone model as the shock wave generator is presented to explain the observed plasma effect on the shock wave. Analysis shows that the plasma spike can effectively deflect the incoming flow before the flow reaches the cone model; such a flow deflection modifies the structure of the shock wave generated by the cone model from conic shape to a curved one. The shock front moves upstream with a larger shock angle, consistent with the experimental results.