Abstract: Using R-wave dispersion phase velocity we can reverse S-wave velocity group of geologic strata. The R-wave method has effective applicability for engineering site because of its characteristics. In detection of the existing foundation Wangjing～Hepingli Railroad, combining the CDP method of multi-trace reflection technology and R-wave spectral analysis technique, we obtain 2-D S-wave velocity profile. According to the coherence between S-wave velocitiy and the value of light dynamic penetration test, we can evaluate the railroad foundation in vertical and horizontal direction.