Abstract: The linear acceleration effect and additional energy divergence of ions by apair of sweeping plates of the pulsing system are investigated. The curves ofadditional energy divergence are given. To overcome the chromatic aberration attarget, which is caused by this energy divergence after the beam pass throughthe Mobley type magnet, magnetic clamps both at entry and exit side of theMobley magnet are suggested. A beam spot of 3×4 mm~2 can be obtained at thetarget for the pulsing system with a pulse width of 1 ns for proton beam.