Abstract: Formation pressure is a key factor controlling hydrocarbon migration and accumulation. Overpressure is widely developed in the Pinghu slope belt of the Xihu Sag in the East China Sea Basin, exhibiting complex distribution patterns that make formation pressure interpretation and evaluation difficult. To accurately identify the origin of overpressure and achieve quantitative assessment, this study comprehensively employed the mudstone compaction curve method, the acoustic velocity-vertical effective stress chart method, and the acoustic velocity-density chart method to identify overpressure in this area as a composite result of undercompaction and fluid expansion. On this basis, an evaluation system combining the equivalent depth (ED) method and the sound velocity rebound (SVR) method newly developed by our research team was adopted to quantitatively evaluate the overpressure with a composite origin. Taking the well W2 as a case study, the error between ED+SVR-calculated pressures and measured values was 0.1%~1.7%, outperforming the Eaton method, which was 0.0%~6.2%. In adjacent wells, the ED+SVR method yielded errors of 1.0%~3.3%, significantly lower than the Eaton method's 2.0%~11.4%. Analysis showed that the Eaton method relied on calibration with measured pressures to determine the Eaton exponent and was sensitive to changes in overpressure origin contributions, making it suitable for single wells or adjacent wells with available measured data and stable origins. In contrast, the ED+SVR method was not limited by these constraints and generally performed well in pressure evaluation for both single and adjacent wells. Its physical significance and reliability were verified by petrophysical and electrical data, demonstrating strong regional applicability. The study clarifies the composite origin mechanism of overpressure in the Pinghu slope belt of the Xihu Sag and establishes the ED+SVR method as an effective tool for quantitative evaluation of overpressure with a composite origin in this area, providing a critical basis for subsequent precise pressure evaluation and exploration deployment in this region.