This work systematically investigated the early hydration process of magnesium oxysulfate (MOS) cement pastes by non-contact impedance measurement (NCIM) and other microstructural tests. According to the evolution of impedance response, the hydration process of MOS cement pastes can be divided into three stages (dissolution, acceleration and hardened stages). The identification of hydration is strongly related to the transformation of solid phases and alkalinity of pastes. The presence and crystalline degrees of 517 phase and 318 phase are evaluated by DTG and XRD tests, respectively. It seems that MgO/MgSO4 ratio does not affect crystalline degree of hydrated products much. MH crystals and cracks are considered to result in the low mechanical strength of MOS cement paste. It can be inferred from experimental results that NCIM is a good method to analyze the hydration evolution of MOS cement pastes in-situ.