As the electromagnetic environment becomes increasingly complex, the problems of traditional carrier frequency measuring are gradually protruded. So the acousto-optic frequency measurement system is designed. Here the optical signal processing is mainly studied. On the basis of acousto-optic diffraction theory, the mechanisms of acousto-optic deflector dealing with frequency signal are described, and spatial filtering of acousto-optic deflector is analyzed. It is obtained that to some degree the system can reduce the effect of noise, because when different frequency signals are input to the piezoelectric transducer, the volume gratings of different grating constants which will have different influence will be established in the acousto-optic media. By experiment, data is tested in the areas of improving signal to noise ratio, ameliorating signal resolution, and enhancing the processing capacity of the transient signal. It is demonstrated that the system has the features of good real-time performance, high interception rate, wide bandwidth, high precision, handling multi-signal simultaneously etc. And it can realize instantaneous frequency measurement of the multiple signals simultaneously arriving at transducer and fully embody the advantages of acousto-optic deflection system.