As the unmanned combat aircraft is one of the current research hotspots, the electromagnetic model A of conventional configuration, model B of tailless flying wing configuration, and model C of triangular wing configuration were established respectively to study the stealth performance of unmanned combat aircraft with different aerodynamic configurations. The RCS (Radar Cross Section) curves of different states were calculated by Physical Optics method, and their characteristics of distribution, frequency and pitch angle response were analyzed. The results show that the electromagnetic scattering characteristics of unmanned combat aircraft are determined by the aerodynamic layout and structure, and are also related to the detail design of leading and trailing edges of wing, fuselage, inlet, nozzle, vertical tail and so on. There are scattering peaks in nose and rear directions of model A, while as the models B and C adopt shape stealth technology, they have no obvious scattering peaks. When the frequency increases, the RCS mean value of each model in different angular domain decreases, and the value of C in nose direction is as low as -34.308 7 dBsm. When the pitch angle changes, the RCS mean value changes oscillatingly, which influenced by configuration design. And configuration C possesses best multi-frequency stealth performance in nose and rear directions, configuration B has better stealth performance in different pitch angles in in nose directions, and configuration A possesses poor stealth performance.