If the upper stage rocket engine, which operates under high altitude environment, is tested on the ground conditions, performance characteristics may not be properly evaluated and causing destruction of the nozzle in severe condition. This diffuser system can simulate high altitude condition on the ground. For the design specification of the diffuser experimentally, a large number of time and costs is consumed by tests changing parameters of operating and geometric conditions. So a numerical study has been conducted to investigate flow characteristics of diffuser for high altitude simulation. At previous study, it had accomplished numerical strategy for the design of the diffuser with nitrogen at room temperature in the non-reacting condition. Properties of burnt gas have been included to check thermal consideration which is the essential design parameter of the diffuser wall. It have been conducted to implement numerical study of the several diffusers, which is selected based on the previous study of flow characteristics with non-reacting conditions, with properties of burnt gas. It have confirmed flow characteristics of the diffuser system. And the baseline diffuser have operated normally in pressure environments upper 42 bar in the same condition at previous study and test. It has also verified the highest wall temperature in the second throat part and diverging part of the baseline diffuser. It have proceeded to study geometric parameters needed at making of the diffuser. Reduction in should be avoided for the accurate simulation of the high altitude. And stable flow characteristics have be observed in the length longer than = 6. In case of =0 mm, the diffuser system have been unstable. So it should be recommended that the length of be fully extend. It have verified that Mach disk is not observed under various length-to-diameter ratios of the second throat part in contrast with non-reacting results. In addition, more stable flow characteristics have been observed and the recirculation zone has not been formed in diverging part with properties of burnt gas. As a result, it have confirmed the influence of properties of burnt gas to check the difference between non-reacting and reacting conditions.