© 2019 Explosive phreatomagmatic eruption is one of the most hazardous eruption styles, particularly in basaltic systems, as the instability of the conduit system can result in a sudden and unexpected shift of eruption style from a mild effusion of lavas to violently explosive activity. The geological investigations on the phreatomagmatic activities in the 7th Century, Suoana-Kazahaya eruption (SKE) of Miyakejima, reveal that the drop of magmatic overpressure in conduit and the distribution of groundwater controlled the occurrence of phreatomagmatic explosion. The “dry” magmatic eruption in the initial phase of the SKE indicates that the positive overpressure of magma in the propagating feeder dike prevents the invasion of external groundwater into the conduit. Explosive phreatomagmatic eruption occurred at the later phase of the SKE from the vents in the higher elevation. The drop of magmatic overpressure in the upper part of the feeder dike caused by the downslope propagation of the eruption fissure allowed groundwater inflow to the still hot and molten feeder dikes. The limited distribution of phreatomagmatic activities indicated the development of shallow groundwater, hosted in the porous pyroclastic deposits within a basin of less-permeable older edifice. As shifts of eruption style from initial magmatic to later phreatomagmatic explosive eruption style in the top of mafic volcanoes are globally observed in many mafic volcanic systems, such as Kilauea and Mt. Etna, this is probably a far more common eruption mechanism, than previously thought, and hence it needs to be considered in volcanic hazard scenario descriptions. The spatial distributions of phreatomagmatic activities in the SKE suggest that the detection of buried caldera structure in a volcanic edifice can indicate a potential site for phreatomagmatic explosion. The monitoring of the propagation of eruption fissure and drops of magmatic pressure and flux can indicate the potential of the phreatomagmatic explosion by the invasion of groundwater into the hot conduit.