Abstract: Computation method for effective path of SAR double bounce scattering from trees based on undulating terrain is presented. The large-scale elements of surface fitting are used to describe the variation of surface slope. Through conditional searching of scattering particles and double bounce scattering, the situation that there may be none or many effective paths of double bounce scattering from trees when the terrain is undulating with large-scale feature is fully considered. In order to improve the computation efficiency, the searching area is restricted and the forest stand is divided into three-dimension grids. The conditional searching for the scattering particles in a certain grid is based on the effective path searching results at the center of the grid. Example simulation of double bounce scattering effective path has been demonstrated based on fractal DEM and measured DEM. Results indicate that, compared with the result based on tilted ground with single slope, the proposed method could effectively characterize the double bounce scattering from trees based on undulating terrain.