Abstract: Two pressure systems are recognized in the North Weifang depression, Shandong. The abnormal underpressure system appears in the northern part of the depression, where the measured formation pressure gradients range between 0.005 and 0.009 MPa/m. The normal pressure system is developed in the southern part of the depression, where the measured formation pressure gradients range between 0.0094 and 0.011 MPa/m.These data have revealed that the formation of the abnormal underpressure system is related to both the uplift and erosion of the Paleogene strata and the decrease of geothermal gradients in the northern part of the depression. The concentration differences and disequilibrium of supply and discharge of the formation water are responsible for the differences in the pressure systems in the northern and southern parts of the depression. These pressure systems have experienced evolutionary processes from overpressures to underpressures and finally to pressure differentiation.