Abstract: Limited mixing layer is the flow that between two different gases formed by the shear effects of the free stream and the jet and affected by the jet wall. In this paper the back step flat plate was used to investigate the limited mixing layer typical flow field structure and cooling efficiency at the high speed and high pressure in the hypersonic wind tunnel. The experimental free stream Mach number was 5, the jet Mach number was 1.28, and the jet total pressure was 0.2~0.7 MPa. Based on the transient schlieren photo system, it was useful to obtain the jet out shock structure changing with the flow nomenclatures by changing the jet total pressure. To obtain the mechanism of the under expansion around the jet outlet, it was necessary to get the regularity of shock system changing with flow nomenclatures. Based on the flow visualization and measurements results, regularity and physical mechanism of flow parameters effects on the cooling effectiveness were obtained. Heat flux distribution and cooling effectiveness experiments were carried out using FRPSP technique in the hypersonic wind tunnel. The relationship of cooling effectiveness and flow condition for flat plate and concave window at 0° was obtained.