The strong explosive eruption of Bezymianny volcano (Klychevskoy volcano group) occurred on 21 December 2017, the preparation phases of which were recorded by a local seismic network. Applying methods of estimation of seismic velocity changes based on cross-correlation of ambient noise, we obtained different velocity variations at several passbands before the eruption. In order to explain the observations obtained, we performed qualitative mechanical estimates of stress changes resulting from the migration of the pressure source. The obtained and estimated velocity changes have the same patterns. We propose that observed and evaluated velocity changes are caused by fluids migration from mid-crustal magma reservoir of the Bezymianny volcano to the shallow magma chamber that then triggered the eruption.