The effection of composite binders based on alkali-activated slag on the immobilization of Sr 2+ were investigated and mechanisms were analyzed, by way of leaching test, SEM and adsorption test. Results show that the ability of the composite binders on the immobilization of Sr 2+ are influenced by the type and amount of alkalis as well as type of mineral admixtures. The effection of the composite binder on the immobilization of Sr 2+ increased with amount of alkalis increased in the range of 3.0% to 6.0% (count as Na2O equivalent). The binder used water glass as alkalis performed better effection on the immobilization of Sr 2+ than that used NaOH as alkalis. In comparison to kaolin and zeolite minerals, binders contain diatomite has best effection on the immobilization of Sr 2+. Reasons lie in that diatomite has the highest ability on the adsorption of Sr 2+ and could improve microstructure of solified substrate effectively.