Abstract: Thermal protection through mass injection is a high-efficiency and active thermal protection technique, which cooling the structure by injecting the stored coolant into the flow-field. The coolant not only absorbs the heats but also effects the flow structure and insulates the heats. The mass injection methods can be used in high heat flux, long-time flying conditions, which is one of the most potential cooling techniques for long-time hypersonic vehicles. The transpiration, film cooling and opposing jet are the typical mass injection techniques for thermal protection of hypersonic vehicles. In this paper, the cooling mechanism of three typical mass injection techniques is introduced. The mode of injection, the characteristics of flow field and the cooling efficiency of three techniques are compared. Therefore, the shortcomings of three techniques are analyzed which prevent them from applying to the vehicles. Several combinational schemes of mass injection techniques for each shortcoming are recommended. Finally, three expectations for further development of the mass injection techniques are proposed. The fluid-thermal-structural coupling method, design and optimization for injection structures and effectiveness evaluation for thermal protection system of the mass injection thermal protection techniques should be developed in future.