For the convenience of studying the influence of serration on the electromagnetic scattering, two aircraft models with straightline and serration mouth inlet are established taking two kinds of flying wing layouts as a basis. The RCS curves with different parameters are calculated numerically by the Physical Optics method. And the effect of serration on scattering distribution and the RCS reduction with different pitch angles frequencies are studied. The numerical results show that the serration mouth inlet can effectively reduce the scattering peaks in nose and rear direction, and also affects the distribution of peak in other angular domains. The RCS reduction decreases in oscillation with amplitude variation from 10~30dB when the pitch angle increases. The RCS reduction varies in oscillation with amplitude variation from 5~25 dB when the frequency increases. The serration has an effects differently on the electromagnetic scattering with different layouts. The stealth performance of layout A in angular domains of 30° in nose direction is improved obviously, and the reduction value is 20~30 dB.