The fiber lasers operating in a radiation environment will suffer from radiation damage, which will reduce the output power and generate radiation-induced heat. This will affect the normal application of the lasers and the safety of the system. In this study, we intend to establish the relation between the radiation environment characterization parameters and the power, thermal effect, and temperature distribution with respect to the gain fiber of the fiber lasers. Subsequently, we conduct a numerical simulation with respect to the power characteristics and thermal effects of the γ-ray radiated double-clad fiber lasers. Additionally, we study the effects of the radiation-induced loss, radiation dose, fiber length, and pumping style on the output power of the lasers. Furthermore, we analyze the thermal effects of the fiber lasers under different pumping styles. The results of this study could provide a reference to study the radiation effects of the fiber lasers.