A label-free lactic acid sensor has been successfully developed by using a Dysprosium single crystal-based photoelectric potential technique via Dy-SCN/FTO electrode. Interestingly, the proposed sensor demonstrated excellent performance for l-lactic acid analysis with a wide linear range of 0.0196∼16.31 mmol/L, the detection limit of as low as 3.20 µmol/L as well as an advisable stability. The feasibility of this strategy was also verified by practical application towards human sweat samples. The mechanism studies indicated that lactic acid molecules specifically bind to the surface of semiconductor materials, which alters the charge distribution of the electrode surface and subsequently results in band bending/photoelectric potential changes. The theoretical formula for this photoelectric chemistry (PEC) strategy was then derived according to charge balance theory. We believe that the proposed Dy-SCN/FTO sensor would open a new way for rapid, non-invasive l-lactic acid level evaluation during human physical condition monitoring.