Using pre-absorbent low activity slag as internal curing material instead of fine aggregate, the autogenous shrinkage and resistivity variation of low activity slag internal curing mortar were studied, the relationship between them was revealed, and the mechanism of low activity slag internal curing was studied from the microscopic point of view. The results show that with the increase of low-activity slag content, the early strength of mortar decreases greatly, but with the extension of age, the strength reduction in the middle and late stages of mortar is smaller than that in the early stage. It is suggested that the appropriate content of low-activity slag is 15%~25% of the fine aggregate quality. The internal curing of low active slag has obvious influence on the resistivity development of mortar. Before setting hardening, the resistivity of mortar increases with the increase of low active slag content. After setting hardening, the slurry resistivity decreases with the increase of low active slag content. The internal curing of low-activity slag can effectively inhibit the autogenous shrinkage at various stages of the slurry, especially in the rapid shrinkage stage and the transient expansion stage. At the same time, the resistivity after 36 h has a good corresponding relationship with autogenous shrinkage, and the change of autogenous shrinkage can be predicted by the development trend of resistivity. The interfacial transition zone of low-activity slag internal curing mortar is more compact and uniform than that of ordinary mortar. The interfacial transition zone of mortar is optimized and the pore structure of mortar matrix is refined. The large pores between 100 nm and 1000 nm are reduced, and the contents of gel pores and small pores are increased. The average pore size is significantly reduced.