The properties of quantum dots are mainly controlled by their density and size parameters, and the nucleation motion of atoms on the substrate determines the density, diameter, height and other parameters of quantum dots. Therefore, studying the diffusion and nucleation process of atoms is important for self-assembly preparation of quantum dots. A study was conducted on the nucleation process of aluminum droplets on GaAs(001) surface during molecular beam epitaxial growth, which leads to the finding that the influencing factors in the nucleation process include the substrate temperature and the deposition rate when other conditions are unchanged. According to the classical nucleation theory, the spatial distribution and geometric structure evolution of the aluminum droplet on the surface of GaAs(001) were analyzed, and the relation equation of the metal aluminum droplet density on the surface with the substrate temperature and the metal aluminum deposition rate was deduced. In the meantime, the minimum atomic number for the three states of the extreme incomplete state, the initially incomplete state and the complete state are calculated to be 1, 2 and 5, respectively.