Bi2O2Se is a promising material for next-generation semiconducting electronics.It exhibits premature metallicity on the intro-duction of a tiny amount of electrons,the physics behind which remains elusive.Here we report on transport and dielectric mea-surements in Bi2O2Se single crystals at various carrier densities.The temperature-dependent resistivity (ρ) indicates a smooth evolution from the semiconducting to the metallic state.The critical concentration for the metal-insulator transition (MIT) to occur is extraordinarily low (nc～1016 cm-3).The relative permittivity of the insulating sample is huge (∈r ≈ 155(10)) and varies slowly with temperature.Combined with the light effective mass,a long effective Bohr radius (a*B ≈ 36(2) nm) is derived,which provides a reasonable interpretation of the metallic prematurity according to Mott\'s criterion for MITs.The high electron mobil-ity (μ) at low temperatures may result from the screening of ionized scattering centers due to the huge ∈r.Our findings shed light on the electron dynamics in two dimensional (2D) Bi2O2Se devices.