This study is aimed at the differences of physical properties such as specific surface area and particle size distribution of cementitious materials. The P·Ⅱ cement, grade Ⅰ and grade Ⅱ fly ash, grade S95 and grade S105 slag were intermixed. Four combinations of fly ash and slag were designed on the premise of fixed cement dosage. The effect of the combination method on the fluidity of the pure slurry was analyzed through the fluidity experiment, and the particle size curve was compared with the Fuller distribution curve. Tests on workability, mechanical properties and free chloride ion concentration of concrete with different combinations were carried out on the basis of the pure slurry mix proportion. The microscopic morphology and chemical composition of hydration product of hardened slurry were analyzed. The effects of particle size distribution of fly ash and slag on the strength and chloride ion penetration resistance of concrete were discussed, and the influence mechanism was revealed. The research results show that the particle size distribution of mineral admixtures is an important factor in determining the quality of particle gradation. The ball effect of grade Ⅰ fly ash is stronger, which makes it play a key role in improving fluidity. The mechanical properties and durability of concrete are affected by the hydration of fly ash and slag, and the degree of pozzolanic reaction. The grade Ⅰ fly ash has larger specific surface area and higher pozzolanic activity. Therefore, the fluidity, workability, compressive strength and chloride ion penetration resistance of grade Ⅰ fly ash and S95/S105 slag combination group are significantly higher than the combination of grade Ⅱ fly ash and S95/S105 slag combination group. Among all the combinations in this experiment, the particle size distribution curve of cementitious material of the combination of grade Ⅰ fly ash and grade S95 slag is the closest to the Fuller curve. The particle size distribution of this combination is the best, the content of Ca(OH)2 in the hydrated slurry is the lowest, and the pozzolanic reaction is the most sufficient. The corresponding concrete has a dense internal structure, and the chloride ion penetration resistance of concrete is good.