Abstract: This paper discusses the frequency resolution of an acoustio-optical spectrometer system under influences of deflector's spectral resolution, coherent light-beam truncation ratio, and photodiode array's response. The broadening of the resoluble line-pattern caused by the response of a photodiode pixel has been computed. According to this discussion the authors proposed the selecting principles for design parameters of the acousto-optical spectrometer including light-beam truncation ratio, focal length of Fourier transform lens, and number of photodiode array elements, i.e. the quantitative criteria for specific observational necessities with an acousto-optical spectrometer considered with resolution, bandwith, and sidelobes of instrumental profile comprehensively. An experimental measurement of the frequency resolution to verify above theoritical computations has also been carried out.