The broadening mechanisms are investigated for the energy spectrum of ions accelerated by electrostatic shocks in overdense plasmas irradiated by intense ultra-short laser pulses. It is found that the width of the shock-accelerated ion spectrum could be ineluctably broadened by three mechanisms, which are the continuously decreasing speed of the shock front due to the energy dissipation into ions, the collisions between the particles, and the further acceleration by the sheath field at the rear of target where when energetic ions arrive. Other effects, such as effect of driving laser pulse duration, are also studied.