We present a detailed characterization of photorefraction in on-chip high-Q lithium niobate (LN) microresonators. We show that the photorefractive effect in these devices exhibits very distinctive temporal relaxation dynamics compared with those in bulk crystals and in mm-sized LN resonators. The relaxation of photorefraction is dominated by a fast time response with a time constant as small as 20.85 ms that is more than three-orders of magnitude faster than those observed in macroscopic devices. The observed fast response of photorefraction is of great potential as a convenient and energy-efficient approach for on-chip all-optical functionalities.