The relation of the fiber amplifier gain and emission cross section and absorption cross section of doped fiber,the thermal performance of the optical fiber amplifier under vacuum environment are theoretically analyzed. Experimental results show that the gain change is 0.0076 dB / ℃ under vacuum environment when the input optical signal is at 1550 nm. When the fiber amplifier works 5 minutes at 55 ℃ under vacuum environment,Er/Yb co-doped fiber temperature drops to 76 ℃ from 106 ℃,the output power fluctuation is less than 10 mW.