Abstract: The West-central sedimentary basin is one of the important areas for oil-gas exploration in Taiwan.Based on the data of oil wells and gravity surveys, the Kriging insertion equations and the depth-density empirical formula, the authors calculated the 3-D density structure and residual density structure of the sedimentary strata of the study area.Moreover, according to the Poisson equation and the FEM, the authors also computed the residual gravity anomaly caused by the 3-D residual density structure.The 3-D density structure shows that the Pleistocene to pre-Miocene sedimentary strata are broadly divided into four density layers at 0 to 6 km depths, i.e.the 2.0-2.30 g/cm³, 2.30-2.45 g/cm³, 2.45-2.60 g/cm³ and 2.60-2.70 g/cm³ layers.There is a relatively large sedimentary depression at 2-4 m km depths in the middle and lower parts of the continental margins of the Taichung basin.The density there is 0.05-0.10 g/cm³ lower than that in the surroundings.The gravity calculation indicates that the residual gravity anomaly produced in the sedimentary strata in the area ranges from-20 to +15 mGal, and that the deep depression corresponds with a trap with a low negative anomaly, whose shape and value are consistent with those and the measured gravity anomaly.