Herein, the influence of gamma-ray radiation on laser power characteristics is investigated experimentally by irradiating the gain fiber of a fiber laser with gamma rays in off- and on-line modes. In the off-line irradiation experiments, it is found that the slope efficiency of the fiber laser remains unchanged when the pump power is larger than a certain value. In the on-line irradiation experiments, with a total radiation dose of 2580 Gy, it is found that the laser output power decreases exponentially during the initial stages of irradiation until it reaches the lowest value, then increases slightly, and thereafter tends to be stable. The variations of power degradation rates with radiation dose under two different irradiation modes are compared, and the power degradation rate of the on-line irradiation laser is found to be higher during the initial stages of irradiation. With the accumulation of radiation dose, the power degradation rates of fiber lasers irradiated in both on-line and off-line modes tend to be consistent. These preliminarily experimental results are explained using color center theory of fiber radiation-induced loss.