Abstract: The pre-saturated low-activity slag was used as a partial replacement of sand for internal curing materials. The mechanical properties and autogenous shrinkage of internal curing mortar with different low-activity slag contents were studied. The microscopic mechanism of low-activity slag internal curing mortar was discussed based on MIP, SEM and XRD test results. The results show that the compressive strength development law of mortar specimens with different low-activity slag contents under standard curing and sealing curing conditions is similar, that is, with the increase of low-active slag content, the early strength of mortar decreases greatly, but with the extension of age, the late strength reduction of mortar is less than early strength reduction. Moreover, low-activity slag can effectively inhibit the autogenous shrinkage of the slurry at various stages, especially in the rapid shrinkage stage and the transient expansion stage. Under the internal curing effect of low-activity slag, although the porosity of the slurry is increased, the average pore diameter is effectively reduced, the distribution ratio of various types of pores is improved, and the pore structure is refined. The hydration product C-S-H gel increases, Ca(OH)₂ is consumed, and the weak structure of the interface transition zone is significantly improved.