Abstract: Residual trend surface analysis on elevation of stratigraphic top surface of prototype basins and the analysis on the geometry of corresponding sedimentary basements indicate that a large flat uplift close to the Caledonian erosional surface exists in the central Ordos Basin.Combining the analysis of syndepositional tectonic deformation in the basements with that of regional tectonic history,ways and directions of the tectonic stress in early and late Yenshan stages are determined.Then the tectonic stress field and strain field of the two stages are simulated by using finite element method,and their genetic relationships to the above mentioned large flat uplift and big gas fields in the central Ordos Basin are studied.It is considered that vertical expansion of Early Paleozoic mudstone and gypsum salt rock layers in compressional stress field might promote the formation of large flat structure trap close to the Caledonian erosional surface where the gas fields are situated.The superimposition of low compressional stress and other factors resulted in negative pressure environment,the central part of the basin became the accumulation area and container for natural gas.