The Kyushu-Palau Ridge (KPR), a remnant arc on the Philippine Sea Plate (PSP), is subducting beneath the Kyushu, southwest Japan. Influenced by the subducting KPR, the Kyushu subduction zone corresponding to the KPR is significantly different from Shikoku subduction zone in terms of gravity anomalies, seismicity, the stress state, and the subducting slab morphology. Significant negative free-air and Bouguer gravity anomalies are observed in a prolonged area of KPR, southeast of the Miyazaki Plain, indicating that this is where KPR overlaps the overriding plate. The gravity anomaly in this area is much lower than that in other areas where the inferred KPR extends, suggesting that the subduction of the buoyant KPR may cause the lower mantle density to decrease. More earthquakes have occurred in Hyuga-nada region where the KPR subducts than in Shikoku forearc and other areas in the Kyushu forearc, indicating that the subduction of the KPR enhances the local coupling between the subducting and overriding plates. The centroid moment tensor (CMT) mechanism of earthquakes shows that stress is concentrated in the accumulated crust beneath the Kyushu forearc corresponding to the KPR, and the shallow thrusting events in the obducting plate are caused by the KPR subduction. The buoyant KPR, with a large volume of low-density sediments, was responsible for the differences of the subduction depth and dip angle of the subducting Philippine Sea (PS) slab between northern Kyushu and Shikoku. The seismic gaps and the sudden change of the dipping angle of the subducting PS slab indicate that slab tear may have occurred along the west side of the KPR beneath southwest Kyushu. A two-tear model was proposed, and the subduction of the buoyant KPR was believed to play an important role in the slab tear.