Abstract: To study the flow characteristics and aerodynamic performance of an over-under TBCC exhaust system during the mode transition process, the unsteady numerical simulation of the mode transition process is completed by using dynamic grid method. Moreover, a series of cold flow wind tunnel tests of the TBCC exhaust system are carried out under several working conditions during the mode transition process, and the results are compared with the numerical simulation results. The results show that the wave structure of the TBCC exhaust flow field is extremely complex during the mode transition process, and the shock generated at the trailing edge of the splitter plate has some effects on the aerodynamic performance of the exhaust system. The axial thrust coefficient keeps above 0.9, yet the lift varies greatly during the process. The static pressure distribution on the walls and schlieren images obtained by wind tunnel experiments are consistent well with the numerical simulation results, which proves the accuracy of the numerical simulation results in this paper.