Volcanic eruptions could be disastrous. Understanding how magma accumulates, migrates, and erupts to the surface has both scientific and societal implications. However, the variation of the scale of volcanoes, the co-occurrence of earthquakes, and the duration of eruptions make it difficult to understand and, eventually, to predict volcanic eruptions, particularly for those without surface deformation. With a method based on ambient noise interferometry, this study characterizes the subsurface response, in terms of the variation of seismic velocities, to different stages of the eruption process at the Great Sitkin Volcano in Central Aleutian Islands. This volcano erupted in May and July of 2021, with elevated seismicity, gas release, and the formation of a new lava dome. It had an increase in seismicity in February 2020 but without any eruption. We measured the variation of seismic velocities from August 2019 to March 2022. We observe a velocity decrease up to one month before the eruption, followed by velocity increases after the eruption. The seismic velocities would restore to the normal level within about one month after the eruption started. We don’t observe a velocity increase before the increase in seismicity rate in February 2020. The observations from seismicity, dv/v, and shear-wave velocity model suggest a four-stage eruption cycle. Despite its remote location and relatively small scale, the findings of this study at the Great Sitkin volcano have significant implications for understanding vocalism and the development and prediction of volcanic eruptions in general.