In order to improve the durability of asphalt mixture under high temperature and rainy weather conditions，the improved GAC-20 mixture is designed by adding 0%, 1%, 2%, 3%, 4% composite high modulus agent to modify the matrix asphalt and optimizing the grading curve, and the road performance of the improved GAC-20 mixture is verified. In addition, the pavement structure performance of the improved GAC-20 mixture is evaluated by composite rutting test and CT scanning test. The results show that the addition of composite high modulus agent can improve the softening point and viscosity of asphalt at 60 ℃, and reduce its penetration, viscosity and ductility at 135 ℃. Considering the basic properties of asphalt and cost control, the content of composite high modulus agent is determined to be 1%. The road performance of the improved GAC-20 mixture can well meet the specification requirements, and compared with GAC-20 mixture, the high temperature stability and water stability are improved by 4.96 times and 1.06 times. The pavement structure with 4cm SMA-13 + 6 cm improved GAC-20 + 8 cm GAC-25 shows good rutting resistance, and the void ratio at the interface is reduced by 0.8% and the internal void ratio is reduced by 0.7%, which effectively improves the compaction effect of asphalt mixture. Therefore, the improved GAC-20 mixture can be used to improve the rutting resistance and water damage resistance of asphalt pavement, which provides a theoretical basis for the study of durable asphalt mixture under high temperature and rainy conditions.