In order to apply the acousto-optic frequency shifter (AOFS) to the optical phase-locked loop for the fast and accurate tuning and to ensure the quality of the signal received by coherent receiver, we introduce the frequency shift principle and the dynamic operation mode of the AOFS in the phase-locked loop, study the insertion loss characteristics of the AOFS systematically, and analyze the effects of power variation of driver and frequency variation in frequency shift process on insertion loss. We design and build a test system for the insertion loss measurement of 1550 nm AOFS based on the beat frequency method. The test results show the variation in the insertion loss under the influence of various modulation signals, and the global dynamic variation trend of insertion loss under multiple influencing factors is also analyzed and verified experimentally. The driving power is optimized according to the experimental results of the AOFS. The optimization results show that the AOFS insertion loss declines by 15 dB in the range of frequency shift when the driving power increases from 25 mW to 1 W, and the maximum relative insertion loss declines by 1 dB.