Abstract: The northwestern segment of the Southeast Indian Ocean exhibits a complex tectonic setting, characterized by asymmetric spreading rates between its two flanks, a remnant subducting slab beneath the southern flank, and mantle plumes to the east. These factors may collectively influence oceanic crustal thickness. To characterize the crustal thickness distribution and investigate the long-term accretion mechanisms in this region, this study focuses on the area between 75°E and 80°E with crustal ages younger than 30 Ma. By integrating gravity inversion and plate reconstruction models, we calculated the age-averaged crustal thickness (AACT) of the oceanic basins on both flanks of the ridge, thereby delineating the spatial variation in crustal thickness. Results reveal a distinct asymmetry in crustal thickness between the two flanks: the northern flank has an average AACT of 6.4 km, being obviously thicker than the 6.0 km observed on the southern flank. Furthermore, both the crustal thickness difference and the half-spreading rate difference between the northern and southern flanks decrease from west to east, while the Curie point depth shows no corresponding systematic variation. Therefore, we concluded that the faster spreading rate of the southern flank is the primary factor leading to its thinner crust.