The Taiwan–Ryukyu arc–trench junction area is a seismic active region and has high potential to generate large earthquakes. However, the detailed deep structures are not fully revealed because of data resolution problems. The Taiwan Integrated Geodynamic Research (TAIGER) project collected two wide-angle and multichannel seismic profiles across the Taiwan mountain belt (T5) and the western end of the Ryukyu arc–trench system (T6). In this study, we perform 2-D gravity modelling for T5 and T6 by taking advantage of well-constrained P-wave velocity models, which helps to reduce the nonunique problem. To better fit the observed gravity anomaly, a block with relatively high-density must exist beneath eastern Taiwan. This high-density material could be serpentinised mantle, which may be resulted from the dehydration of the subducting Eurasian plate. Also, our result indicates that the subducting Gagua Ridge has a deep crustal root and extends northward to the Ryukyu forearc area.