Abstract: To reveal the influence mechanism of activated coal gangue powder (ACGP) on the sulfate resistance performance of coal gangue aggregate concrete (GAC), specimens with different ACGP replacement ratios (0%, 15%, 30%) were designed. The evolution of mass, relative dynamic elastic modulus, and compressive strength corrosion resistance coefficient of these specimens under sulfate attack coupled with dry-wet cycling were investigated. The degradation mechanisms were elucidated through microstructural characterization. The results indicate that the compactness and strength of GAC specimens improved during the initial sulfate erosion stage, while expansive corrosion products caused pore expansion and crack propagation in later stages. ACGP effectively filled pores in GAC and promoted the formation of hydration products, significantly enhancing sulfate resistance. Specifically, the compressive strength corrosion resistance coefficient of GAC with 30% ACGP replacement reached 0.84, markedly higher than that of the control group (0.34). The proposed prediction model accurately describes the performance evolution of GAC with low ACGP replacement ratios under sulfate attack, demonstrating strong correlation with experimental data.