Themechanism of negative ion extraction under real conditions with the complexmagnetic field is studied by using the 3D PIC simulation code. The extraction region of the negative ion source for the negative ion based neutral beam injection system in fusion reactors ismodelled. It is shown that the E x B drift of electrons is caused by themagnetic filter and the electron suppressionmagnetic field, and the resultant asymmetry of the plasmameniscus. Furthermore, it is indicated that that the asymmetry of the plasmameniscus results in the asymmetry of negative ion beam profile including the beam halo. It could be demonstrated theoretically that the E x B drift is not significantly weakened by the elastic collisions of the electrons with neutral particles.