In recent years, the basic physical properties and potential applications of semiconductor quantum dots, especially InAs quantum dots, have been extensively studied. Many researchers use the structural changes of InAs quantum dots to modulate their photoelectric properties. Various amount of indium (3 ML, 4 ML, 5 ML) were deposited on GaAs(001) surface by droplets epitaxy for investigating the nucleation mechanisms and surface diffusion of indium. As the amount of indium deposition increases, the droplet size (including diameter and height) increases obviously. Not only that, higher deposition amount also leads to higher density of droplets at the same substrate temperature. The critical thickness of indium droplet formation was theoretically calculated as 0.57 ML on the GaAs (001) surface. It is in agreement with experimental result. The mechanism of the formation and morphology evolution of droplets is explained by diffusion and intermixing between the gallium and indium migration. The critical thickness of In droplet obtained in the experiment and the nucleation mechanism of In droplet on GaAs(001) can provide experimental guidance for InAs quantum dots growth.