The relation curves of reaction of acoustic and optical wave under the momentum matching condition are presented. It is found that, in the scope of visible and infrared spectrum range, only when the angle between the incident ray and the crystal optical axis is about 56°, there exists a unique acoustic wave vector, which enables two situations [o (the ordinary ray) in e (the extraordinary ray) out and e in o out] to satisfy the parallel tangent momentum matching condition, and the wavelength of two diffraction rays equals each other. This specific angle is labeled as the equivalent point in theoretical design of acousto optic tunable filter (AOTF). This result has been proved through experiments. Applying equivalent point theory in designing AOTF can improve the signal to noise ratio of spectrum measurement significantly without sacrificing any wavelength resolution.