The acousto-optic tunable filter (AOTF) is one kind of prismatic elements which is developed by the abnormal acousto-optic interaction principle, it has wide applications in many fields. The spectral resolution is one of the key index for evaluating the performance of the acousto-optic tunable filter. We have analyzed the factors that were connected with the spectral resolution. And we have pointed out that the improvement of the spectral resolution was limited through optimizing the design parameters of a single AOTF. A new method called “double filtering” that can effectively enhance the spectral resolution is presented. The principle of double filtering method is described. The main technology of improving the spectral resolution by double filtering method is discussed. The feasibility and the advantage of this method on the spectral resolution improvement are confirmed. The double filtering method provides a new idea for the improvement of the spectral resolution.