A numerical simulation study is conducted on the flow field characteristics and laws of energy transfer of the high-gradient mixing layer in a rocket-assisted ramjet engine （RARE） . The study focuses on the effects of incoming flow parameters， such as the rocket-side Mach number （1.2—2.0） and total temperature ratio （2—4）， on the dimensionless thickness variation of the mixing layer， as well as the kinetic and thermal energy transfer laws. The research results indicate that after 50 mm of mixing between the two fluid streams， the mixing layer has already reached a self-similar mode. The variation in the total temperature ratio has a more pronounced effect on the dimensionless growth rate of the mixing layer. The exchange of kinetic energy between the two fluids primarily occurs within the mixing layer. Heat can transfer from the unmixed rocket filed， across the mixing layer， to the unmixed ramjet filed. The heat transfer from the rocket to the ramjet combustion chamber mainly depends on the total temperature ratio on both sides. When the total temperature ratio increases from 2 to 4， the static enthalpy increase on the ramjet side is nearly fourfold.