MgO slurry was prepared by using light-burnt MgO as raw material and aluminum lactate, magnesium lactate and calcium lactate as additives. The hydration process of MgO was systematically characterized by hydration heat test and ionic conductivity analysis. The effects of different lactate salts on the setting time, microstructure of hydration products and hardening strength of MgO paste were studied. The results show that lactate significantly inhibits the hydration process of MgO in the early stage, but has little effect on the hydration process in the later stage. The introduction of lactate not only prolongs the setting time of MgO paste, but also improves the hardening strength of MgO paste. The hardening strength of the sample with aluminum lactate is 24% higher than that of the sample without admixture. Lactate has a significant effect on the microscopic morphology of MgO hydration products. The hydration products of the samples with aluminum lactate show irregular petal-like structure with large specific surface area. The hydration products of the samples with magnesium lactate show regular and interlaced petal-like structure, while the hydration products of the samples with calcium lactate show spherical petal-like structure.