A post‐discharge peak in both extracted negative ion current and negative ion density has been observed in volume hydrogen negative ion sources. This peak is closely related to the principle of the pulsed volume source. To understand the physical mechanisms involved, we have studied the post‐discharge peak in a medium‐sized negative ion source with a numerical simulation code. The simulation shows a post‐discharge peak in H−density, which is most important at high discharge current and low gas pressure. The post‐discharge peak in H−density which we find is principally due to the reduced destruction of H−by electron detachment. The peak size and duration is limited by the decay of plasma densityneand plasma potentialVp. The decay ofnerapidly reduces H−production by dissociative attachment, and the decay ofVpreduces the electrostatic confinement of H−. We find that electron energy loss is principally due to the energy carried out of the plasma by lost electrons. In this casene,Te, andVpall decay on the ion confinement time scale, which determines the length of the H−post‐discharge peak. A significantly longer ion confinement time would increase the length and magnitude of the H−peak.