To get known the influence on the RCS of front sector which is arisen by weak scatterers such as gaps of different width, stair heights and structure types including straight gaps and zigzag gaps, and to emphasize the importance of decreasing weak scatterers, the distributing status and characteristics of weak scattering sources like surface gaps and steps on the conventional vehicle are introduced. The radar backscatter strength of the gap is theoretically analyzed. The low RCS carrier and RCS testing models are designed, on which certain RCS experiments of gaps with different width, stair heights and architectures are carried on. The test results indicates that in order to control the surface electromagnetic defects (SEMD) of stealthy vehicles, it should take some measures such as reducing gap width and step height, replacing straight gaps by zigzag gaps, etc, which are proved to be high-effective technology ways.