The plugging performance and degradation properties of temporary plugging agents remain critical challenges in diverting fracturing technology for deep, high-temperature reservoirs. Conventional particle–fiber systems are constrained by strict size-matching requirements and poor thermal stability, often failing to achieve reliable plugging in complex, high-temperature reservoirs. To overcome these limitations, this study presents a gel–fiber composite plugging strategy based on in situ gelation. This method employs a pre-gel solution as a carrier for fibers, with elevated temperatures inducing in situ polymerization to form a fiber-reinforced gel. Results demonstrate that the optimized pre-gel solution maintains low viscosity during surface preparation and wellbore flow, while rapidly gelling within 0.33 h at 180 °C, ensuring good injectability and controllable gelation. The incorporation of fibers significantly enhances the mechanical properties of the gel, increasing the elastic modulus from 180 Pa to over 3300 Pa and maintaining stability under cyclic shear strains ranging from 10% to 200%. In simulated fracture sealing experiments, the composite system achieved a sealing pressure gradient exceeding 49 MPa·m−1 for 2–3 mm fractures, representing a 2.8-fold increase compared to the pure gel system. Moreover, all components degrade into low-viscosity fluids under high-temperature conditions, facilitating flowback. This study provides a novel temporary plugging strategy for deep, high-temperature unconventional reservoirs, featuring high strength, degradability, and broad adaptability for efficient diverting fracturing.