In an oil exploitation process, hydrogel plugging agents can effectively reduce the water-oil intermixing, decrease water extraction volume, and enhance oil recovery rate. The practical applications of traditional polyacrylamide (PAM) hydrogel plugging agents in oilfield are limited by their non-biodegradability, poor mechanical performance, and inferior temperature-resistance. This work developed a mechanically stable and high-temperature-resistant composite hydrogel (STP) by incorporating biodegradable scleroglucan (Slg) and TEMPO-oxidized cellulose nanofibers (TOCN) in the PAM hydrogel. The addition of Slg conferred heat resistance to the PAM hydrogel, while TOCN reinforced the mechanical strength. Anti-aging analyses revealed that the STP endured for 108 h in a saline environment at 140 °C. In the water flooding characterization, the STP displayed a breakthrough pressure of 42.10 psi/ft. at a flow rate of 0.75 cm/min. Under these extreme conditions, the plugging pressure reached 14.74 psi/ft., meeting the essential criteria for oilfield water plugging. This research demonstrates the potential of polysaccharides in the preparation of sustainable, tough, and heat-resistant water plugging materials.