Abstract: Impacts of backstreaming ions on the focus of single-pulse and multi-pulse high-current relativistic electron beams were studied by solving the envelop equation. The result indicates that smaller integrated spot size could be achieved by shorten the focal distance in single pulse machine, while in multi-pulse machine, electron beam pulses would be disrupted after interaction with the long ion column. The feasibility of using foils to block the back streaming ions mechanically was also studied by both numerical simulation and experimental methods. Simulation of foil expansion after interaction with electron beam shows that although the expansion is considerable during a time scale of 1.06 μs, there is still enough material to stop the ions. Faraday cup experiment at Dragon Ⅰ linear induct