Inhibition effect of different volume of ground slag （GSL） on alkaline silica reaction of field active aggregate was studied, and compared with normal mortar. The results show that the inhibiting effect enhanced with increasing ground slag, and while the added volume is up to and more than 50%, the expansion rate of the mortar specimens is less than 0.10% after its curing time of 28 days. The inhibition mechanism of ground slag on the alkaline silica reaction is well expounded by press filtration test and energy dispersive spectroscopy (EDS) analysis. That is, the increase of groud slag can make the content of K+ and Na+ in pore solution and the ratio of n(Ca) to n(Si) in C-S-H Gel decrease.