Abstract: A preliminary study on the aerodynamic design of the combined cycle nozzle working in the range Ma=0~6.0 with small length to height ratio was carried out under strong geometric restricts. The line of the nozzle was designed by the method of characteristic. The effects of the design Mach number, the three-dimensional lateral expansion angle and the relative position of the two nozzles on the aerodynamic performance were studied. A nozzle aerodynamic design scheme considering both the effective utilization of space and the aerodynamic performance was presented. The numerical simulation results show that reducing the Mach number at the design points can improve the performance of the combined cycle nozzles during the subsonic flight and avoid serious overexpansion of the nozzles. With the increase of the lateral expansion angle, the aerodynamic performance of the nozzle that keeps the exit height unchanged at high Mach number is superior, while the aerodynamic performance of the nozzle at low Mach number decreases seriously. The relative position of the turbine engine and the ramjet nozzle outlet has a great influence on the aerodynamic performance of the transition point, and there is an optimal position layout, which achieves the optimal thrust performance. The thrust coefficient of the combined cycle nozzle is about 0.920 at the designed Mach number. The flow field is smooth transition during the transition mode, when the thrust coefficient is not less than 0.918.