For the space applications of fiber amplifier， the radiation performance of fiber amplifier in radiation environment was investigated experimentally. The evolution laws of output power and spectral character are investigated experimentally by irradiating the gain fiber of an erbium-ytterbium co-doped fiber amplifier with gamma rays. The investigation of the noise characteristics of erbium-ytterbium co-doped fiber amplifier has been carried out by means of frequency spectrum measurement. In the on-line irradiation experiments， with a total radiation dose of 50 krad， it is found that the laser output power， the power of peak wavelength and optical noise decrease with the increment of radiation dose. The noise characteristics of the fiber amplifier are analyzed by the optical noise model after irradiation. Compared with the results before irradiation， the coefficients of relative intensity noise of relaxation oscillation noise and the intermediate frequency part are increased by 1.625×10^-4 nW·mW^-2·Hz^-1 and 3.122×10^-4 pW·mW^-2·Hz^-1， respectively， while the coefficients of shot noise are reduced by 0.900 pW·mW^-1·Hz^-1 and 0.035 pW·mW^-1·Hz^-1， respectively. Therefore， it is necessary to pay attention to the suppression of the relative intensity noise for the practical application of fiber amplifier in space.