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To address the problems of rubber powder (RP) modified asphalt, such as segregation, high viscosity, and poor storage stability at high temperatures, this study reports the preparation of gallic acid/ rubber powder (GA/RP) modifier by grafting gallic acid (GA) on the surface of RP. GA/RP was, in turn, composited with SBS modified asphalt (SBS-MA) under the synergistic effect of microalgae bio-oil (MB) to produce GA/MBRP/SBS composite modified asphalt (GA/MBRP/SBS-CMA). In the test results of asphalt's three indicators, the optimum dose of GA surface-grafted RP was attained at a GA-to-RP ratio of 20 g:1.5 g. The dynamic shear rheometer (DSR) test revealed that the storage modulus (G') and loss modulus (G") of GA/MBRP/SBS-3 modified asphalt were 48.0% and 51.5% higher than those of the RP/SBS-CMA (64 °C and 10 Hz), respectively. Segregation tests showed that the difference between the upper and lower softening points of modified asphalt was reduced, indicating that the phase separation of GA/MBRP/SBS-CMA improved. Fluorescence microscope (FM) tests revealed that RP and SBS were fully swollen and formed a well-developed network structure responsible for the enhanced mechanical properties of GA/MBCR/SBS-CMA under high-temperature conditions. Compared to the RP/SBS-CMA, GA/MBRP/SBS-CMA exhibited better storage stability and strong resistance to segregation. These results indicate that GA/MBRP/SBS-CMA has practical application value in the construction industry due to its more potent mechanical properties and storage stability. [ABSTRACT FROM AUTHOR]