Abstract: The neutral beam injection (NBI) is a very successful and effective heating method in large Tokamak nuclear fusion device. The cooling of extraction system and back panel of arc chamber limits the development of the high current ion source which is the key part of NBI system. In this paper, the magnetic field of the backstream electron dump plate permanent magnets was simplified as flaring magnetic field. So the movement of backstream electrons can be simplified as helical movement of charged particle. The simulation of heat load of dump plate caused by backstream electrons was done by means of the simplified model, which makes a good reference for optimizing the long pulse extraction system.