The time evolution of the cross-shock electrostatic potential jump during one specific high Mach number quasiparallel shock reformation cycle is examined via a 1D hybrid simulation.It is shown that when the average value of the electrostatic potential jump is low,its instant value can be large so that the shock is in a favorable profile to directly reflect the incident ions.Our simulation also suggests that,for the ones that finally become injected ions (which can be further accelerated by diffusive shock acceleration mechanism),the first step of their reflections should be mainly attributed to the electrostatic potential jump,instead of the magnetic force,as stated in some previous studies.