The formation mechanism of porous anodic oxides remains unclear till now. The classical field-assisted dissolution (FAD) theory cannot explain the relationship between the current-curve and FAD reaction. Few papers have studied the influence of the electrode potential. Here, the innovative “electrode potential theory”, “oxygen bubble model” and the ionic current and electronic current theories were introduced to explain the growth of porous anodic oxides and the current-time curves. Take the anodization of Ti in aqueous solution containing NH4F as an example, the electrode potential was calculated in detail and the morphology of porous anodic oxides was explained clearly at the low voltages. The morphology is determined by the ratio of the ionic current and the electronic current. Here, different metals are classified into two groups: compact oxide layer metal (easy to form the compact oxide layer) and releasing oxygen metal (easy to form oxygen bubbles). The electrolyte is also classified into two groups for their nature: compact oxide layer-assisted electrolyte and releasing oxygen-assisted electrolyte. This study establishes a preliminary system for the formation of anodic oxides from different metals in different electrolytes, which helps to better understand the formation mechanism of porous anodic oxides in different metals.