Abstract: Ion back bombardment is believed to be a major cause of degradation of the quantum efficiency (QE) of the photocathode in direct current(DC) high voltage guns and will reduce the operational lifetime of the cathode. With particle tracking and QE mapping, the effects and limitations of the beam off-axis emission to suppress the ions striking the photo-emission region are studied. The simulation results suggest that the ions mainly bombard the electrostatic center (EC) and the flux of ions near the location of beam emission is reduced by 95% with the beam off-axis emission. The QE maps on the cathode before and after the DC gun operation with CW beam demonstrate that the QE decays seriously within a large region near the EC and the QE degradation in the photo-emission region is suppressed by the beam off-axis emission. However, the off-axis emission will also degrade the beam quality. With an off-axis emission of 6 mm from the EC, the simulation results show that the transverse emittance is increased by about 4mmmrad.