Abstract: To enhance the high-temperature performance and deformation resistance of road asphalt, this study utilized agricultural waste rice husk biochar (RHB) in combination with SBS to prepare a composite modified asphalt, and systematically investigated its rheological properties. With the SBS content fixed at 4%, the RHB content was set at 10%, 12%, 14%, and 16%. Temperature sweep tests, MSCR tests, and BBR tests were conducted to evaluate the viscoelastic behavior of the composite modified asphalt under different proportions. The results showed that when the RHB content reached 16%, the overall rheological performance of the composite modified asphalt was optimal. As the RHB content increased, the complex modulus significantly increased, while the phase angle decreased. At 16% RHB content, the rutting factor at 70 °C was approximately 50% higher than that of SBS-modified asphalt, indicating superior high-temperature stability and rutting resistance. The MSCR test results further revealed that the non-recoverable creep compliance decreased markedly and the percent recovery increased significantly. Specifically, at 16% RHB content, the non-recoverable creep compliance was reduced by an average of 77.3%, and the percent recovery increased by 37.3% compared with SBS-modified asphalt, demonstrating a stronger resistance to permanent deformation. However, the BBR test results indicated a slight decrease in low-temperature crack resistance. Overall, the RHB–SBS composite modification system can effectively enhance the high-temperature rheological properties of asphalt, although its low-temperature ductility is slightly affected.