Detection of super-low altitude target is a hot point in current military confrontation. Multipath Interference is a difficult problem nowadays. To solve the problem,'G-Brewster' is put forward from electromagnetic scattering. The elements of rough surface are thought about and the best angle is found to detect the target. To get the echo ,the target is meshed and PO+MEC is used to calculate the scattering field, and then the reflection coefficient is derived. On the basis, multipath echoes are gotten. The condition of the existence of G-Brewster is studied out, and the angle that strength of multipath interference is lest is found out. The discipline of G-Brewster on environment is posted. The study will lay a foundation of super-low altitude target.