As a method for light- filtering, acousto- optic tunable filter (AOTF) has wide application prospect. Design and manufacture of high-performance AOTF are key part of this tech. In this work, based on the momentum mismatch, the diffracted efficiency of an AOTF is calculated. Also the relations between momentum mismatch and band-pass and aperture are investigated. The results demonstrated that the tune relation of an AOTF changed with the incident angle of light and the diffracted wavelength is decided by acoustic wave at a fixed incident angle of light. Otherwise, the aperture and band-pass of an AOTF changed with the incident wavelength; we can acquire narrow band-pass and big aperture with a proper incident angle. The theoretical calculations are in agreement with experimental results. This research will be helpful for the design of an AOTF with good performance.