To investigate effect of plenum chamber total pressure on arcjet thruster operation, numerical simulation of operation under various plenum chamber total pressures was performed on the basis of mechanism analysis on its operation and modeling for chemical non-equilibrium flow and steady state electromagnetic field within it. Various flow mechanisms and interactions between electromagnetic field and high temperature ionized gas were covered in the fluid dynamic and electromagnetic equations, and various impact reactions were taken into account in the chemical dynamics model. Non-oscillatory and non-free-parameters dissipative (NND)finite difference scheme (a total variation diminishing scheme) with second order accuracy was adopted to solve the fluid dynamic equations, a finite rate chemical reaction model was developed to calculate ingredient producing rate, and an ADI over relaxation iteration technique was used to solve the electromagnetic discretized equations. Parameter distributions within the thruster and its macroscopical performance were presented. The investigation shows that plenum chambertotal pressure has multifarious effects on arcjet thruster operation, and both specific impulse and thrust efficiency can be improved by increasing it befittingly under steady arc.