Abstract: A composite rough surface model is presented for describing the concave structure of valley environment.The electromagnetic scattering characteristics of valley composite random rough surface are studied. Monte Carlo method is used to build valley composite random rough surface, Gauss spectral function is used to simulate ground part of valley, and Pierson Moskowitz (PM) spectral function is used to simulate sea part of valley. Weighted inverse tangent function is presented to smooth adjacent region. The ray density normalization (RDN) theory is introduced to improve the SBR (shooting and bouncing ray) method. In consideration of the inartificial cavity construction of valley in natural environment, the improved SBR/ PO is used to calculate the electromagnetic scattering of the valley composite rough surface. Electromagnetic scattering characteristics of ground, sea and valley composite random rough surface are compared. The impacts of open area of valley concave structure on the curves of bistatic scattering coefficient (BSC) of horizontal-to-horizontal (HH) polarization of valley environment are analyzed. The influences of root mean square (RMS) and correlation length of ground rough surface on BSC of HH polarization are discussed. The numerical results can be used as reference in the fields of remote sensing, detection and inversion of electromagnetic environment.