Recently, gravity waves and two-dimensional turbulence have been proposed as possible causes for the observed mesoscale wind fluctuations in the atmosphere. In order to use wind velocity data measured by MST radars to investigate atmospheric gravity wave spectra, it is important to apply the correct relation between the observed wind fluctuation spectrum and the wave spectrum. In this paper, based on acoustic-gravity wave theory and radar observation principle, this relationship is quantitatively derived for the general case of acoustic-gravity waves. Examples are given to show how the theoretical results can be used to interpret the observational data as well as to derive parameters for the model wave spectrum. From the theoretical results, tests for the consistency of the gravity wave model for atmospheric wind fluctuations will be proposed.