Abstract: The middle?shallow reservoir of Nanyishan oilfield in Qaidam basin is characterized by the coexistence of abnormal low pressure, normal pressure and weak abnormal overpressure. Aiming at the characteristics, this paper presents the constitutive model and feature for the fluid pressure compartment of such a reservoir in this area. The study shows good reservoir sealing ability in the reservoir, which is also reflected by its light crude oil, low viscosity, saturated hydrocarbon value of m(ΣnC21-)/m(ΣnC22+). It is suggested from the multi?well section with predicted and measured pressure coefficients that the abnormal low pressure and normal pressure?weak abnormal overpressure fluid compartment is characterized by vertical superimposition and along?bedding distribution, where local abnormal overpressure belt occurs near the faults due to differential fluid charging in the abnormal low pressure compartment. The constitutive model for the compartment is as follows: the bottom is the Upper Ganchaigou source rocks with capillary pressure and abnormal pressure dual sealing mechanism; the top is the Upper Youshashan formation with saline sediments of shale and marl interbedded with gypsum layers; while the edge is shaped by the gradual changes into such a compartment from thrust fault interface with lateral sealing, rock facies and reservoir physical properties