A method and a sensor for detecting L-arginine are provided. The method includes synthesizing ferrocene-functionalized hexadecapeptide dithiocyclopentane (FC-P16 Peptide), preparing a polypeptide composite membrane-modified electrode (FC-P16 Peptide/AuE), detecting L-Arg and other steps. The results show that the polypeptide composite membrane-modified electrode (FC-P16 Peptide/AuE) exhibits excellent electrochemical response properties to L-Arg. In 10 mmol/L phosphate-buffered saline (PBS, pH=7.4), the DPV response peak current of the polypeptide composite membrane-modified electrode has an excellent linear relationship with the L-Arg concentration of 1.0×10−13 mol/L to 1.0×10−7 mol/L, with a detection limit of 1.0×10−13 mol/L. With prominent reproducibility, repeatability and selectivity, the modified electrode has potential application in life science and nutritional health.