Abstract: The turbine based combined cycle (TBCC) inlet was designed based on the thrust and mass-flow requirement of the TBCC propulsion system. The design points parameters of TBCC inlet, namely the captured area and the mode transition Mach number from turbine mode to ramjet mode were investigated through one-dimensional analysis of turbine engine and ramjet engine in this paper. Then the TBCC inlet scheme which can operate from Ma 0 to 4 was designed and its mode transition device was designed based on the translation scheme. The variable scheme of the TBCC inlet can be achieved through the single degree of freedom mechanism and the profile of the inlet operating at different Mach numbers was given. After the investigation of the length, control point values and the area diffusing rules of rectangular-to-circular shape diffuser of turbine flowpath, the numerical simulation results indicate that the diffuser length, control point values and the area diffusing rules have mild effects on the total pressure recovery and Mach number at the exit of the turbine flowpath, but control point values have significant effects on the distortion index. According to the results above, the diffuser length is chosen to be 3m, the value of the control point to be 1.5 and the area changing rule along the diffuser is chosen as a rapid turning at the entrance. The total pressure recovery of the TBCC inlet is 0.45 at Ma4.0 and 0.79 at Ma2.2, the distortion index is 0.15 at Ma2.2.