Numerous research has been conducted on starch oxidation, but few reports have described it quantitatively. The oxidation of soluble starch (SS) can be quantitatively determined using a single‐current‐step method, where the aldehyde group content of oxidized starch approaches the maximum value at the voltage jump point. To verify the feasibility of the method, SS electrooxidation and electrochemical synthesis with gelatin (G) are observed and analyzed with reference to voltage changes. As an experimental application, the electrochemical synthesis of SS‐G at varying pH is compared to that of a physical mixture SS+G. According to the changes in the electrosynthesis and differences in the structure and properties of the synthesized products, the single‐current‐step method can be used to distinguish online.