A spray painting tool for painting key caps is disclosed, which comprises: a platform, having a plurality of columns formed on a top surface of the same; a panel, having a plurality of holes formed thereon at positions corresponding to the plural columns for allowing the plural columns to pass through the panel therefrom; a diaphragm, disposed on the panel while aligning a plurality of holes formed thereon with the plural columns for allowing the plural columns to pass through the diaphragm therefrom; and at least a holding unit, each being arranged on the panel at a position corresponding to one of the plural column for fixedly securing a key cap. As the diaphragm can be easily replaced, the spray painting tool can be used repetitively without worrying the stacking of residue paint on its panel. Moreover, as the spray painting tool is simple in structure and ease to use, painting key caps can be achieved with lower cost and less time.