For stealth aircrafts and tanks, the discontinuous characters such as gaps are important for RCS(Radar Cross Section) reduction and control. Basing on CFIE (Combined Field Integral Equation), a program using MLFMA (Multilevel Fast Multi-pole Algorithm) is developed. The bistatic RCS of metal sphere by Mie series solution and the experimental curves of a multi-gap plate are compared with computational curves to validate the correctness of this program which shows that the program can be used in investigating the scattering from discontinuous characters in aircraft surface. The relation of single-gap, three-gap and incoming frequency is researched by MLFMA. When the incoming frequency is greater, numerical results show that the electromagnetic scattering enhances and the RCS arithmetic average increases for single-gap and multi-gap; when the incoming angle is small, the average value increases fast, for single-gap plate, the curves are similar for different frequencies and the second crest of curve moves far away from the major crest, scattering from multi-gap has coupling performance and the curves change sharply beside the characteristic of single-gap plate. The coupling performance is gained by the superposition of different phases of three single-gaps, when frequency increases, the position of scattering superposition changes, at the same time, the effects of couple and surge will be obvious. This conclusion can find its application in the design of reducing the RCS of scattering from the gaps in aircraft surface and thus improve the stealth performance of aircraft.