Abstract: The middle section of the Solonker-Xar Moron suture and its surrounding areas have successively been affected by the closure of the Paleo-Asian Ocean during the Paleozoic, the closure of the Mongolian-Okhotsk Ocean during the Mesozoic, and the subduction of the Pacific Ocean plate since the Meso-Cenozoic, resulting in very complex, deep structures in this area. We conducted a regional-scale three-dimensional S-wave velocity tomographic study of the area. To do this, we used surface waves from earthquake data and ambient noise data recorded in recent years. The lithospheric tomographic model shows that the regional crustal thickness does not vary considerably, but the crust in the Songliao and the Lower Liaohe basins is thinner than that in the Daxing'an Mountains. The fact that the crustal thickness remains consistent with changes in topography indicates that these areas have reached gravitational equilibrium. However, the crustal thickness of the Liaodong uplift and the Bohai Bay Basin is inversely related with topography, indicating that these areas may still be in a tectonically active state. At depths greater than 80 km, the S-wave velocities spread in an east-west direction approximately parallel to the Solonker-Xar Moron suture. The complexity of the upper-mantle velocity distribution to the north of the Solonker-Xar Moron suture differs significantly from that to its south. Furthermore, south of the Solonker-Xar Moron suture, the complexity of the velocity distribution in the east differs considerably from that in the west. This implies that the destruction resulting from the Pacific Ocean subduction system varies significantly within the study region, and remnants of the paleo-Asian ocean slab may still exist in the south.