To investigate the flow and drag characteristics of the ramjet flowpath of a hybrid over-under turbo-based combine cycle（TBCC）， TBCC engine under transonic flight conditions， a high-Mach aircraft model with a cruise Mach number of 4.0 based on hybrid over-under TBCC engine was designed， and its performance in Ma_∞=0.7—1.6， H_∞=11 km to simulate the aircraft-engine internal/external flow and its coupling characteristics in the flight environment. The results show that in the whole transonic state， the static pressure of the air pressurized at the inlet of the ramjet inlet reaches 85%—90% of the free flow stagnation pressure， and the air flow is close to the stagnation state， indicating that there is a strong throttling flow effect in the combined inlet， and the throat of the ramjet flowpath is the main contributor to the throttling effect of the combined inlet port. The exhaust flow of the ramjet tail nozzle is simultaneously interfered by the aircraft bypass flow and the exhaust system of the turbine channel， and the turbine channel exhaust jet has various interference mechanisms such as expansion compression and exhaust injection to the airflow of the ramjet tail nozzle itself. The drag analysis shows that the differential-pressure drag coefficient is two orders of magnitude higher than the friction drag coefficient of the inner surface， which is the main source of the drag of the ramjet engine under the transonic state. In the subsonic state， the inlet port drag accounts for 60%—80%， which is the main drag component of the ramjet engine， in the Ma_∞>1.0 supersonic state， the inlet port drag ratio decreases with the flight Mach number and decreasing gradually， and the drag of the tail nozzle increases rapidly， indicating that the drag contribution gradually shifts to the tail nozzle， and the two tend to approach.