Polyacrylamide is a commonly used chemical agent in oil and gas fields, widely applied in enhanced oil recovery (EOR) techniques. However, with the increased exploitation of high-temperature oil and gas reservoirs, the thermal stability of polyacrylamide gels has become insufficient, significantly affecting the development of oil and gas fields. Previous studies have shown that organic–inorganic composite crosslinked gel systems can improve thermal stability. To further enhance the performance of this gel system, nano-scale graphite powder was incorporated into the gel. The results indicate that the addition of graphite powder further improves the gel system’s performance, with a gelation time of 14.0 to 17.0 h and achieving Class I strength. After aging at 140 °C for 120 days, the gel still exhibits good performance without dehydration or degradation. Yield stress experiments demonstrate that the addition of graphite powder does not excessively affect the yield stress of the gel base liquid, thereby maintaining the ability of the polyacrylamide base liquid to migrate into deeper layers of the formation. Furthermore, graphite powder can significantly increase the yield stress of the gel after it has formed, ensuring its stable presence within the formation without being displaced. As the concentration of graphite powder increased, the gels demonstrated improved resistance to deformation during the creep phase and enhanced recovery during the recovery phase. The organic–inorganic composite cross-linked gel system with added graphite powder retains the original gel’s sphere-tree structure; however, the tree-like structure becomes flattened compared to the original gel. This flattened tree-like structure has better mechanical properties, which helps enhance the gel’s strength. When the graphite powder mass fraction ranges between 0.5 wt.% and 2.5 wt.%, the differential scanning calorimetry (DSC) results show that the peak temperature gradually increases from 157 °C to 182 °C, indicating that adding graphite powder can further improve the gel system’s thermal stability. Compared to materials like graphene, graphite powder is more cost-effective, offering a new technology for enhancing oil recovery in high-temperature reservoirs.