Based on the principles of acousto-optic deflection and coherent optical detection,a frequency measurement system is established.In order to improve the efficiency of space optical coherent detection,a method that signal and reference beams are respectively injected into corresponding polarization-maintaining(PM)fibers before combining is proposed.Meanwhile,how to use collimating and focusing lens to ensure the injection efficiency of the signal beam over the whole frequency band of operation is discussed in detail.Experimental results show that the system can achieve accurate and real-time frequency measurement,and its interception rate reach to 100% within the frequency bandwidth of measurement.