In order to control the length of tunnels within Al foil etched in HCl-H2SO4 solutions, the influence of on-off control of the DC on growth and passivation of tunnels has been investigated. From SEM of oxide replicas of tunnels, it was found that, in a given etchant solution at a special temperature, the longest tunnel length depended only on the turn-on interval of DC, and the number of pits was determined by the total electricity of the DC. The corresponding mechanism is that the potential of Al foil changed rapidly at the point of the switch of DC by the result according to the anodic polarization curve and potential-time (E-t) response curves. The moment the DC was switched on, the potential increased immediately over pitting potential, leading to nucleation of pits at the surface and growth of tunnels at special lengths. When the DC was switched off, the potential rapidly decreased to a passive state, leading to the cessation of nucleation and the death of tunnels. Therefore, the longest tunnel length can be controlled and a non-piercing layer can be obtained. Furthermore, consequent etching of Al foil by the on-off control of the DC is beneficial for maintaining a good mechanical strength.