Conventional diffuser is introduced briefly and its mechanism is analyzed. Full Navier-Stokes equations based on low magnetic Reynolds number assumption with Favre weighted average mass are used to simulate the flow field in supersonic diffuser with added electric and magnetic field. Compared with conventional diffuser, it is obvious to see that the deceleration and expansion are realized in shorter distance when proper electric and magnetic field added, and part of the kinetic energy is transformed into electric energy. Study shows that the positions of the first shock wave, and the second shock wave and the length of shock train in the diffuser when certain magnetic field added are respectively 62.19%, 77.74% and 50.18% to the ones of conventional diffuser.