Abstract: Aiming at the long-term protection effect of aluminum alloy sacrificial anode on the steel shell of immersed tunnel in the complex medium environment of backfill stone and sea water, a 1∶80 physical scale model was established, and the accelerated electrolysis method was used to simulate the protection performance under different service cycles. The protection potential distribution, dielectric resistivity change and anodic dissolution morphology of steel shell were systematically tested and analyzed. The results show that the initial steel shell protective potential was below -1.0 V (vs. Ag/AgCl/seawater electrode, same below), and gradually shifted positively with the extension of service time. The average protective potential at the end was -0.91 V, but it always met the cathodic protection criterion for a potential below -800 mV in the whole cycle (100 a). With the extension of service time, the environmental resistivity of the medium increased significantly, from 66 Ω·cm at the initial stage to 146 Ω·cm at the end stage. The sacrificial anode showed stable electrochemical performance. The final potential remained negative at -1.05 V, and the dissolution morphology was basically uniform. However, due to the reduction of anode size and the increase of medium resistivity, the current decreased from 40 mA to 11 mA.