The PmP' wave group with long offset has the characteristics of flat geometry, low apparent velocity and above the zero line of travel time in the seismic record section with the observation length of more than 250 km or more in the deep seismic sounding data of the western Cathaysia block. Because it is connected to the PmP reflected wave group and the waveform is similar, the conventional wave group identification is often used as a part of the PmP wave group. Using this wave group as part of the PmP wave group for theoretical seismogram simulation, it is difficult to obtain satisfactory fitting of travel time and amplitude at the same time. Taking the far-offset PmP' wave group of the wide-angle reflection/refraction profile SP3 of the Lianping—Heyuan—Haifeng earthquake in eastern Guangdong in 2021 as an example, the apparent velocity at the farthest end of the wave group is calculated to be 6.04 km/s, which is less than the average P wave velocity of the crust calculated by the travel time of the PmP wave group in the critical region of 6.27 km/s, indicating that the wave does not have the property of PmP wave. A new lower crust and Moho interface model is used to fit the two-dimensional theoretical seismograms of the PmP' wave group. The results show that this wave is a refracted wave from the lower crust, indicating that the lower crust of the Western Cathaysia block generally has a positive velocity gradient structure. This reflects the result of magma intrusion since the Mesozoic era in the southeastern margin of South China, which may be the direct evidence of newly crust.