To address the challenge of excessive water production in high-temperature and high-salinity gas reservoirs, an in situ self-emulsifying nanoparticle-enhanced active oil-selective water control agent has been developed. To investigate the mechanism of its efficient water control performance, a high-temperature and high-pressure micro-shear emulsification method simulating reservoir conditions was employed to evaluate the dynamic/static structural stability of the generated Pickering emulsion. The synergistic effects among different components of the water control system were also examined. The results indicate that the interaction between nanoparticles and emulsifiers promotes in situ emulsification, producing a W/O Pickering emulsion with high internal phase characteristics, temperature and salt resistance, and solid-like structural properties. Additionally, the highly stable three-dimensional structure formed by interconnected nanoparticles within the continuous oil phase endows the blocking system with strong dynamic structural stability. Rheological curves demonstrate that during water phase blockage by the Pickering emulsion, the initial negative effect of structural rupture in the continuous phase caused by deformation is counteracted by a simultaneous increase in viscosity, thereby enhancing the limiting strain for stable water control capacity. The findings provide deeper insights into the in situ emulsification process and the water control mechanism of Pickering emulsions, establishing a theoretical foundation for further improvements in Pickering emulsion technology.