Abstract: In road engineering, higher requirements are being placed on the high-temperature deformation resistance, moisture stability, and low-temperature cracking resistance of asphalt mixtures. As a renewable agricultural by-product, rice husk biochar (RHB) possesses both a high specific surface area and favorable thermal stability. When combined with SBS for composite modification, it has the potential to synergistically enhance pavement performance while promoting sustainable construction. To this end, this study systematically evaluates the overall performance of RHB/SBS composite-modified asphalt mixtures. Mixture specimens prepared with the optimal blend ratio were tested for high-temperature deformation resistance using the rutting test (60°C), while moisture stability and low-temperature cracking resistance were assessed through the immersion Marshall test and freeze–thaw splitting test, respectively, thereby providing a comprehensive evaluation of pavement performance. The results showed that, in the rutting test at 60°C, the dynamic stability of the composite-modified mixture increased by approximately 46% compared with SBS-modified asphalt and by about 78% compared with RHB-modified asphalt. The residual stability from the immersion Marshall test reached 91.7%, and the freeze–thaw splitting strength ratio was 89.7%, demonstrating excellent resistance to moisture damage. Although the low-temperature splitting strain was slightly lower than that of the SBS group, it still satisfied the requirements of current specifications. Overall, the combination of RHB and SBS produces a synergistic modification effect, significantly enhancing the high-temperature stability and moisture resistance of asphalt mixtures while maintaining adequate low-temperature cracking resistance. This provides a feasible approach for the development of high-performance and eco-friendly pavement materials.