Acrylamide and 2-acrylamido-2-methylpropanesulfonic acid (AM-AMPS 25) and water-soluble phenolic resin (WSPR) were used to develop a gel plugging agent with appropriate gelation time, long-term thermal stability, and low cost for high-temperature and high-salt reservoirs (130 °C, 41.529 g/L). Although AM-AMPS is a temperature and salt-resistant polymer, after crosslinking with WSPR, the thermal stability of the formed gel is far from meeting the requirements. The gel undergoes terrible syneresis when aged for 30 d and their dehydration rate is over 95%. Whereas the same gel with additional 1% nanoparticles exhibits much better thermal stability, shows no syneresis and their dehydration rate is less than 1%. Therefore, the effect of nanoparticles on the gelation properties of the gel and their stability-enhancing mechanism were investigated. Dynamic light scattering (DLS), Cryo-SEM and viscosity change analyses were applied to polymer solutions before and after the addition of nanoparticles, as well as Cryo-SEM analysis was applied before and after the addition of nanoparticles. The results reveal that nanoparticles could improve the viscosity retention of polymer solutions, decrease the hydrodynamic radius of polymer chains, and reduce the heterogeneity of entanglement between polymer molecular chains, resulting in a more homogeneous distribution of polymers in solution and therefore a more uniform mesh structure of gel is formed, thus the thermal stability and water holding capacity of gels at high temperature and high salinity are improved.