The design, fabricating, performance perameters and typical application of a polarization maintaining fiber coupled acoustoopticl frequency shifter (AOFS) for low frequency shifting is introduced in this paper. The doublecrystal reversecascading design method, i.e. the difference values between the operating frequencies of two AO crystals is used as the frequencyshifting frequency, is used to overcome the problem that a single crystal is difficult to achieve low frequency shifting. The performance parameters of the sample are as followa: working wavelength of 1 053 nm, frequency shift of 10 MHz, insert loss of 3.3 dB, rise time of 33 ns and linear polarization output.