L-cysteine, a component of the symmetric L-cystine, is essential in numerous biological activities. Thus, detecting cysteine rapidly, selectively, and sensitively is of tremendous interest. Herein, g-C3N4@CdS composites were employed as sensing elements in a photoelectrochemical platform for L-cysteine sensing. In this system, g-C3N4@CdS composites provided much better optoelectronic function than bare CdS materials owing to their high photon-to-current conversion efficiency and excellent anti-photocorrosion properties. The innovative photoelectrochemical sensor has a wide determination range of 5 to 190 µM, a very low detection limit of 1.56 µM, a fast response time, and good long-term stability (ca. 1 month). Without applying any separation procedures, a low concentration of CySH was successfully detected in human urine samples, which is compatible with the results of chemiluminescence.