Abstract: The MC-SNICS ion source of NEC compact AMS system at the AMS laboratory of Peking University was upgraded recently. Spherical ionizer and improved Cs reservoir were equipped with this upgraded ion source. 12C - beam current up to 150 μA can be obtained with careful optimization of operating parameters. Increased beam current causes bigger ripple of high voltage of tandem. To enlarge the diameter of the aperture downstream the analyzing magnet from 10 mm to 12 mm is helpful to keep the stability of the measurement. Experiment results show that background do not increase obviously with larger aperture.