Abstract: The mechanism of mineral admixtures on suppressing alkali-silica reaction has been studied through analyzing the pore solution and observing the surface of aggregates in mortar bars with scanning electron microscope(SEM).The results show that when mineral admixtures are used in the concrete, not only the alkali concentration in the pore solution can be reduced, but also the diffusion of alkali ions from the outside of concrete to the inside can be prevented, and the diffusion of alkali ions from the pore solution to the surface of the aggregates can also be prevented.So the attack of alkalis to aggregates is reduced, and the alkali-silica reaction is suppressed.