The optic-acoustic reaction relationship is analyzed to noncollinear acousto-optic tunable filter (AOTF) under normal phase matching condition in this paper. To the generally designed AOTF, the diffracted o and e rays have different wavelengths. The two rays cannot be used simultaneously in practice. In order to magnify the intensity of light that can be used, the two diffracted rays should have same wavelength. The main research content of the paper is how to get two rays with same wavelength in normally produced AOTF crystal. For the AOTF crystal not produced by equivalent point theory, the incident light can be deflected a small degree with the normal of the crystal plane. By normal phase match condition it can be found that certain degree can diffracted the same wavelength rays. Experiments have validated the method. The deflected degree is called equivalent offset angle. For different diffracted wavelengths, there are corresponding different equivalent offset angles. Finally the variation rule of equivalent offset angle with diffracted wavelength is given.