Porous anodic alumina thin films with rainbow rings were fabricated in oxalic acid electrolyte by one-step electrochemical oxidation of Al sheet. The hole depth and aperture diminish symmetrically outward from the center of the films. The relationship between the density of rainbow rings to the oxidation voltage and the time in oxalic acid solution are discussed. Theoretical study on the formation mechanism of porous anodic alumina thin films with rainbow rings indicates that the effect of carbon rod is similar to carbon dot electrode in the process of anode electrochemical reaction. Result of the theoretical study is consistent with the experimental phenomenology.