Abstract: The performance of magnetohydrodynamics(MHD) energy bypass scramjet was analyzed using quasi-one dimensional mathematical model. MHD application on ideal and un-ideal scramjet was investigated, the results show that for ideal scramjet there is no need to use MHD devices, for un-ideal scramjet the performance can be improved by the use of MHD energy bypass. Considering about the engineering constraints, such as combustion chamber entry Mach number and the maximum allowable temperature of the entire channel, the performance of scramjet and magneto-plasma-chemical(MPCE) was compared, results show that MHD energy bypass can broaden the flight Mach number range of the engine and improve performance when the vehicle is flying at un-design Mach number or the engine is not perfectly designed. The influence of some important parameters such as load factors and MHD channel pressure coefficients to the performance of the MPCE was analyzed, results show that the optimization of these parameters can increase the specific impulse of the engine but the degree is constrained by the engineering conditions.