[en] According to the characteristics of high current negative ion beam extraction and acceleration system in the negative ion-based neutral beam injector, a numerical simulation model and calculation code for the negative ion beam system are established. The effects of electro-magnetic field configuration, geometry, plasma parameters, beam density, and negative ion stripping losses on the negative ion beam characteristics are investigated. The initial optimized results of the 800 keV negative ion beam system with 8 electrodes show that the magnetic field of this system can deflect the extracted electron beam to the extraction electrode as an electron acceptor at lower energy, and assuming that extracted H^- negative ion beam density is 21 mA·cm^-2 and H^- initial temperature is 0.2 eV, divergence angle of 85% output beamlets is 0.234 degree