This paper analyses the mechanism which backstreaming ions are produced and the effect of backstreaming ions on focus of electron beams. The methods which control the effect of backstreaming ions are also given. The results show that if backstreaming ions are H+ or C+, they seriously affect the focus of electronic beams after H+ or C+ backstreams 60ns; if backstreaming ions are Ta+ or W+, they do almost not affect the focus of electronic beams when the beam pulse is short, but seriously destroy the effect of the beam focus when the beam pulse > 200ns.