In this paper, active MgO was used as the alkali activator, and the early hydration kinetics and microstructure development of MgO-activated slag were explored by means of the hydration heat test, X-ray diffraction (XRD), thermogravimetry (TG-DTG), Fourier transform infrared spectroscopy (FT-IR), mercury intrusion porosimetry (MIP), and scanning electron microscopy–energy dispersive spectrometry (SEM-EDS). Three types of MgO were selected based on reaction time (R-MgO<M-MgO<S-MgO). The other two variables in this study include MgO content and curing age. The research results show that S-MgO is more suitable as an alkali activator and the total hydration heat of S-MgO-activated slag is much lower. The second exothermic peak is more obvious with the increase of MgO content. The main early hydration products of S-MgO-activated slag were a hydrotalcite-like phase, C-S-H gels, and C-A-S-H gels; the early main hydration product of R-MgO-activated slag was brucite. With the increase of MgO content, the total porosity decreases, i.e., the total porosity of the S-MgO specimen is the smallest, followed by the M-MgO specimen, and the total porosity of the R-MgO specimen is the largest. With the increase of S-MgO content, the processes of crystal nucleation and crystal growth are accelerated. When the S-MgO content is 20% by weight, the phase boundary reaction process and diffusion process of the MgO-activated slag system accelerates, which is more conducive to the diffusion, recombination, and precipitation of hydration products. This study provides an experimental and theoretical basis for the use of green alkali activator.