The drying shrinkage characteristics of alkali slag cement mortar from the view of irreversibility are studied. It is found that the shrinkage of alkali slag cement mortar water curing after drying is significantly greater than that of OPC mortar which is cured under the same conditions. The drying shrinkage (14 d) of alkali slag cement mortar activated with NaOH solution and water glass is respectively 5.5 and 2.2 times of those of OPC mortar. Moreover, the majority of drying shrinkage of alkali slag cement mortar is irreversible, and the proportion of this part is up to 86%(activated with water glass) and 68%(activated with NaOH solution).