An atomic layer deposition system is provided, including: a main body, a platform, a gas distribution showerhead assembly and a first ring member. The main body defines a reaction chamber, and the platform is located in the reaction chamber. The gas distribution showerhead assembly is disposed on the main body and includes at least one gas inlet channel and at least one gas diffusion plate. Each of the at least one gas diffusion plate includes a plurality of through holes. The first ring member defines a radial direction and is disposed between the platform and the at least one gas diffusion plate. A region of the at least one gas diffusion plate distributed with the plurality of through holes defines an outermost distribution profile. An inner circumferential wall of the first ring member and the outermost distribution profile keep a distance in the radial direction.