Photodarkening (PD) is recognized as an important factor that affects the reliability and lifetime of many fiber lasers and amplifiers. Temporal characteristics of the fiber laser and a self-recovery phenomenon with time dependence in darkened fibers are observed. A feasible method to measure PD is designed and several domestic fibers are tested. A 638 nm laser diode (LD) is utilized to bleach the darkened fiber. Experimental result indicates that the excess loss caused by PD can be considerably mitigated by 638 nm irradiation and one reasonable explanation is demonstrated.