The focusing ability of the image charges on annular beams is analyzed, and the image charge is on the conducting foils which are placed perpendicular to the beam axis. The given calculating results can be employed to select the geometrical parameters of the foils for annular beam focusing. The first image charge focusing relativistic klystron amplifier experiment was done to study the effects of the foils on the bunching process. Although the transparancy of the foils to the beam is only 0.88, the results show that the foils do not affect the beam bunching process.