Based on the mechanism of alkali silica reaction(ASR), the effective alkali was defined as the part of alkali that had diffused into the reactive aggregate and participated in the chemical reaction. Using glass aggregate as reactive aggregate, the influence of curing temperature, reactive aggregate size and reactive aggregate addition(by mass) on the effective alkali content(by mass) in single reactive aggregate particle was studied. Based on the Ficks first law and Ficks second law, a calculation method for effective alkali concentration was proposed. The results show that the effective alkali content is influenced by curing temperature obviously. However, the influence of reactive aggregate size on effective alkali content is not remarkable. Besides, the effective alkali content decreases with the increase of reactive aggregate addition. The proposed calculation method for effective alkali concentration can be used to evaluate the effect of curing temperature, reactive aggregate size and addition effectively.