Abstract: To reveal the subduction process of the western Pacific Plate and its impact on the overlying plate, this study utilizes data from the NECsaids-III broadband seismic array deployed in Northeastern China (the Erenhot–Shuangliao profile) and extracts reflected body wave signals from the mantle transition zone (MTZ) using the ambient noise cross-correlation method. A common reflection point stacked profile is constructed, and by identifying and tracking the reflected P-wave phases from the upper and lower boundaries of the MTZ (P410P and P660P), the lateral variations of the MTZ across the north-south gravity lineament (NSGL) are obtained. The results indicate that the P410P and P660P phases along the profile exhibit clear segmentation. Near the NSGL, the phases are relatively chaotic and difficult to track, while on both sides, the continuity is better, with multiple strong reflection segments. Specifically, beneath the Erenhot Basin and the Greater Khingan Range region, the phases are clear and stable, with slight fluctuations in their arrival times. The deviations from the reference two-way travel time are, on average, no more than ±1 s, corresponding to depth variations of less than 5 km. Beneath the southern Songliao Basin, the phases are advanced and delayed by approximately 4 s, respectively, relative to the reference two-way travel time, indicating a significant thickening of the MTZ by over 30 km. Combined with existing seismic images, it is inferred that the significant thickening of the MTZ and the presence of high-velocity body within it beneath the southern Songliao Basin are related to the stagnation of the subducted western Pacific Plate in the MTZ of this region. In contrast, the high-velocity bodies within or just above the MTZ beneath the Greater Khingan Range and the Erenhot Basin have negligible impact on the MTZ discontinuities, suggesting that these high-velocity bodies differ significantly in nature from the stagnant oceanic plate and are likely derived from the delamination of the overlying continental lithosphere.