As an important part of the thermal protection system of solid rocket motors, flexible thermal insulation materials need to have the characteristics of light weight, ablation resistance, and high mechanical properties. In recent years, significant progress has been made in the application research of fibers and their surface modification technologies in flexible thermal insulation materials. The introduction of fibers enhances the structural integrity of the high-temperature carbonized structure and the ablation resistance of flexible thermal insulation materials. Combined with fiber surface modification technology, it effectively strengthens the interfacial interaction between the fibers and the material matrix, significantly improving the comprehensive properties of the materials. This article reviewed the research and application of fibers and their surface modification methods in flexible thermal insulation materials in recent years, including the filling of different flexible thermal insulation materials with inorganic and organic fibers, as well as the effects of fibers on the ablation performance and mechanical properties of flexible thermal insulation materials before and after modification. At the same time, it summarized and analyzed the research focuses of fibers and their surface modification in flexible thermal insulation materials.