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Nanomaterial-enhanced hydrogels have attracted considerable attention as temporary plugging agents for lost circulation control due to their improved strength under high temperature and high pressure (HTHP) conditions compared to conventional hydrogels. In this study, a pseudointerpenetrating network nanocomposite hydrogel (PINNG), which was composed of temperature-resistant polyacrylamide (TSPAM), polyethyleneimine (PEI) and cellulose nanofibrils (CNFs), was proposed. CNF with a fibrous structure exhibited a more significant effect on the rheological behavior and mechanical strength of the hydrogel than nanosilica (nano-SiO 2). The reversible CNF network endowed the gelant with increased zero-shear viscosity and significant thixotropy. Furthermore, the presence of CNF noticeably improved the strength of PINNG, and the elastic modulus of PINNG was 1232% greater than that of the blank sample. According to analyses of Fourier transform infrared spectroscopy (FTIR), thermogravimetry-derivative thermogravimetry (TG–DTG) and scanning electron microscopy (SEM), it can be deduced that the strengthening mechanism of CNF was mainly related to physical filling, physical crosslinking, and especially the formation of a pseudointerpenetrating network structure with the crosslinked polymer chains. Due to the strengthening effect of CNF, PINNG exhibited a better pressure-bearing performance than the blank sample and the nano-SiO 2 composite sample under 140 °C for the core with 2 mm of fracture width. Additionally, the favorable degradability of the PINNG system demonstrated that PINNG exhibited great potential in the application of lost circulation control as a temporary plugging agent. [Display omitted] [ABSTRACT FROM AUTHOR]