A rotatable fastening device includes a rotatable base and an impact member. The rotatable base rotates along a pivoting axis, and the rotatable base is removably connected with and driving a rotatable member. The impact member is removably connected to an outside of the rotatable base, and the impact member has at least one gravity unit projected outwardly therefrom. The gravity unit is driven by the rotatable base to rotate around the pivoting axis for generating a tangent impact force.