Abstract: A preliminary study was carried out on the interface between ramjet and scramjet modules of dual combustor ramjet.Efforts were focused on the influence of the design parameters on the mixing performance using numerical method.The design parameters included the height of the step, the step geometry and the proportion of the mass flow of ramjet modules.An interface with better mixing performance was presented in the paper.Results indicate: the Mach number at the duct exit is 1.48, the total pressure recovery is 0.765, pressure ratio is 17.71 and temperature ratio is 4.05 for the free stream of the Mach number is 4.For the Mach number is 6 condition the Mach number at the duct exit, the total pressure recovery, pressure ratio and temperature ratio are 2.08, 0.502, 28.03 and 4.67 respectively.