Abstract: The accelerating tube is one of the most important parts in high current implanter which provides the desired energy and focusing for ion beam.The factors affecting focus characteristic in high current implanter with three gap acceleration tube are discussed. focusing degrees of different energy ion beam are computed, and the electric field required to prevent beam expansion due to space charge effect are analyzed. The beam envelope inside the three acceleration gap shows a decrease of the beam radius with the increase of the accelerating voltage ratio up to the optimal value. Beyond this optimal value the beam lines make a crossover with the axis of the accelerating tube.