The invention discloses a common middle point (CMP) detection method used for ground penetrating radar (GPR). The detection method comprises the steps that the maximum offset and offset control intervals of the GPR are determined; detection is performed many times to collect detection data, wherein the offset is set to be zero during detection for the first time, and then the offset is increased based on the offset control intervals every time before detection until reaching the maximum offset; a GPR time cross-section is formed based on the collected detection data, an equi-directional axis tracking line of radar waves is indicated on the time cross-section to obtain the slope value of the equi-directional axis tracking line, and then the speed of an underground medium layer and a corresponding underground medium dielectric constant are calculated based on given formulas. By utilizing the detection method, the speed of the underground medium layer can be detected and calculated accurately.