Polymer gel systems have been widely applied to control excessive water and improve oil recovery (IOR) in petroleum reservoirs. They are usually divided into two main types, in-situ cross-linked polymer gels, and pre-formed polymer gels. In recent years, nuclear magnetic resonance (NMR) technology has been gradually applied to the research of polymer gel systems due to its unique analysis advantages. This paper is intent to review these works systematically. For in-situ cross-linked polymer gel systems, NMR can be used to characterize the chemical structure changes of the polymer, the cross-linker, and the auxiliary agent in the formulation of the polymer gel systems. Moreover, the gelation time and the gel strength of the in-situ cross-linked polymer gel systems can also be measured by NMR. For pre-formed polymer gels, NMR can be employed to detect the chemical structure of the designed products. Last, the NMR method can evaluate the plugging, water control, and oil improvement performance of the polymer gels in porous media without using dopants. This review can help readers build a more systematic understanding of the application of NMR technology in polymer gel systems for IOR and help researchers to more deeply study the performance of polymer gel systems.