In this study, a pretreatment method for improving the radiation resistance of Er-Yb co-doped silica fiber (EYDF) is proposed. EYDF is the object in this method and is processed by two steps, including deuterium loading and pre-irradiation. The effects of pretreatment conditions on the laser performance and radiation resistance of EYDF were systematically studied. An online irradiation experiment setup was utilized to evaluate the radiation resistance of EYDF. The results demonstrate that the pretreatment can significantly improve the radiation resistance of EYDF, with minimal impact on the laser output power and slope efficiency. Specifically, the radiation-induced gain variations in the pristine fiber and the pretreated fiber with a cumulative dose of 240 krad were 3.13 dB and 1.81 dB, respectively. Additionally, the high-vacuum experiments show that the proposed pretreatment method can maintain a long-term stable radiation resistance improvement in the fiber. This study provides a method to improve the radiation resistance of EYDF for space applications.