Turbo diffuser for over/under type TBCC (turbine based combined cycle engine) inlet was designed with extended center line under different flow section area variation laws and blend radius variation laws. The influence of design parameters on the turbo diffuser and its aerodynamic characteristics in turbo mode were investigated by numerical simulation. High speed wind tunnel experiments were carried out to validate the numerical simulation method. Results indicate that turbo diffuser obtains high total pressure recovery coefficient and low total pressure distortion index at outlet when the ordinate of center line control point is within 1.50-2.25 and the ratio of constant area section length of turbo diffuser to exit radius is within 0.3-0.7. Both the flow section area and blend radius variation laws in a "first quick

then slow" manner contribute to a low total pressure distortion index at outlet of turbo diffuser. With the increase of flight Mach number

mass flow coefficients of both inlet and turbo diffuser firstly decrease until reaching its lowest point at flight Mach number of 0.9

then increase subsequently. Total pressure recovery coefficient at outlet of turbo diffuser firstly increase then decrease and the peak comes around at flight Mach number of 0.7. The total pressure distortion index at outlet of turbo diffuser is less than 0.5 for turbo mode and it satisfies the requirements of turbo engine for the flow quality at the entrance section.