A thickness asymmetric electrode structure on an oxygen-terminated type IIa diamond was designed and prepared (one electrode was semitransparent to ultraviolet light and the other blocked the transmission of ultraviolet light). This structure exhibited an apparent photo-induced rectification property under irradiation by a deuterium lamp. This is attributed to the mechanism by which the light penetrating the electrode reduces the metal–diamond contact barrier. Furthermore, we developed a light-modulated Schottky barrier diamond photodetector based on this mechanism. Solar-blind light can lower the Schottky barrier height in situ in the presence of light, which significantly enhances the photocurrent. However, the Schottky barrier is not reduced by light regulation when there is no light; therefore, the low dark current of the detector is still guaranteed. Compared with the non-photo-regulated Schottky barrier detector, the photo-regulated Schottky barrier detector exhibits a 128% increase in responsivity at 220 nm under a 1.6 V/μm bias. For such an obvious difference in detection performance, this mechanism has rarely been a focus of studies on diamond detectors. In addition to diamond detectors, light-modulated barrier technology can also be applied to other fields related to the diamond surface potential, such as color center control and Schottky diodes; it can also be used to control or evaluate device performance.