The compressive strength development of NaOH and water glass activated slag mortar cured at 5℃ was studied. Bulk resistivity, selective dissolution, mercury intrusion porosimetry(MIP), scanning electron microscopy(SEM) were used to analyze and characterize the low temperature hydration processes and early microstructures of alkali activated slag binder(AAS). The results show that alkali activated slag binder can hydrate at 5℃, while the rates of compressive strength development and hydration are lower than that of samples cured at standard curing condition. Under the same conditions, the compressive strength of mortar activated by water glass is higher, but the bulk resistivity and hydration degree are lower than those activated by NaOH. When samples are cured at 5℃ for 1d, the porosity and macropores increase, while weak hydration product layers and large amounts of unreacted tiny slag particles are found in hardened paste, which leads to a slow growth rate of early strength.