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Polymer hydrogels have been proven effective in lost circulation control in well drilling. However, common polymer hydrogels suffer from unfavorable strength and stability under high-temperature and high-pressure conditions, which makes the hydrogels difficult to meet the needs of well drilling in high-temperature, deep, and ultradeep reservoirs. In this study, a nanocomposite gel (ISNG) system composed of polyacrylamide (PAM), polyethyleneimine (PEI), cetyltrimethylammonium bromide (CTAB), and nanosilica (nano-SiO 2 ) was proposed. The strength of ISNG consisting of 7.0 wt % PAM, 1.0 wt % PEI, 1.0 wt % nano-SiO 2 , and 0.3 wt % CTAB was approximately 10.3 times higher than that of the blank sample. The superior strength endowed ISNG with favorable pressure-bearing performance, and the breakthrough pressure of ISNG achieved 0.17 MPa under 120 °C for the core with a length of 50 mm and a fracture width of 1.0 mm, while the breakthrough pressure of the blank sample was only 0.08 MPa. The Fourier transform infrared (FTIR) spectroscopy and thermogravimetry−derivative thermogravimetry (TG−DTG) analyses confirmed that the superior strength of ISNG compared to other samples was closely related to physical interaction and its network structure. The results of dynamic light scattering (DLS), zeta potential (ζ-potential), scanning electron microscopy (SEM), and rheology tests showed that the strengthening mechanism for ISNG was mainly attributed to the in situ self-assembled nanocomposite network of PAM, nano-SiO 2 , and CTAB. The in situ surface modification of nano-SiO 2 by CTAB changed the interaction between PAM and nano-SiO 2 , which resulted in variations in the hydrodynamic volume of the network. The maximum strength of ISNG was achieved at 0.3 wt % CTAB concentration due to the optimum network structure. The in situ self-assembled nanocomposite network enhanced the physical cross-linking density of ISNG without damaging the chemical cross-linking reaction between PAM and PEI.