In this work, we design and fabricate a compact photoelectrochemical (PEC) sensor by integrating a graphene-MoS2 heterostructure on an optical fiber tip. The graphene serves as a transparent carrier transport layer, and the MoS2 presents a photoelectrical transducer that generates photocarriers and interacts with ascorbic acid (AA) in solution. This device is used to demonstrate a self-powered detection of AA with a concentration range between 1 mM and 50 mM, and a time response of ∼ 6 ms. The device downsizes traditional PEC systems to the micrometer scale, benefiting the real-time monitoring of biochemical changes in small areas and opening the pathway for miniaturized PEC sensing applications.