Abstract: In order to explore the influence of coal gangue on the performance of concrete, C30 concrete was prepared with different contents of coal gangue powder（CGP） and different substitution rates of coal gangue coarse aggregate（CGA）. The flexural properties and chloride ion penetration resistance of coal gangue concrete were studied. The influence law and improvement mechanism were further analyzed by mercury intrusion method and scanning electron microscope. The results showed that with the increase of dosage, the flexural strength of coal gangue concrete increased first and then decreased, and 15 % CGP was the optimal dosage of flexural strength. The resistance to chloride ion penetration increased with the increase of CGP content. When the substitution rate of coal gangue coarse aggregate was 100 %, the incorporation of CGP had a more significant optimization of its flexural strength. The addition of CGP increased the harmless pores such as gel pores and transition pores, and decreased the harmful pores such as capillary pores and macropores, thus optimizing the particle size distribution of concrete and improving the structural compactness. On the one hand, CGP produced a large amount of C-S-H gel in the secondary hydration, which made the microstructure of coal gangue concrete more dense. On the other hand, the unreacted CGP can be filled into the pores and cracks of coal gangue concrete to improve the strength of the interfacial transition zone, and then improve the flexural strength and chloride ion penetration resistance of coal gangue concrete.