Abstract: A magnetic mirror device and neutral beam injection method is provided. The neutral beam is obliquely injected at a higher magnetic field position rather than at the midplane of magnetic mirror device. This method is beneficial for confining a higher density of fast ions at the turning point zone with a higher magnetic field, as well as obtaining a higher mirror ratio by reducing magnetic field intensity at middle plane rather than increasing the maximum magnetic field intensity of the magnetic mirror device. It increases a fusion power density of the fast ions reaction and reduces the end loss power plasma, consequently, higher fusion energy gain can be achieved.