Acousto-Optic Tunable Filter as a new beam splitting elements of spectral imaging. We often regard the diffraction center wavelength which is corresponded with the incident light perpendicular to the incident plane of AOTF as the wavelength of CCD spectral measurement. However, the different positions of target have different incident angles when the incident light enter into the AOTF, which will cause error when the CCD actual spectral measurement is compared with the spectral measurement of vertical incidence. The characteristics of this system we have used is that the target light imaging on the focal plane of CCD by passing the pre optical system，AOTF and the imaging lens, which have achieved the goal that the target light imaging on the whole system only once. Compared with the system of secondary imaging, the system of first imaging can improve the quality of image and the light energy utilization ratio. Because of the field angle of AOTF is ±3°, we analyzed the regular pattern of the diffraction wavelength changing with angle of incidence and obtained the revised spectrum measurement equation by processing the real measured value of diffraction wavelength changing with angle of incidence. The experimental results show that its relative error can be reduced an order of magnitude by using the revised equation to measure the spectrum. This method can lay the foundation for improving the measurement accuracy of AOTF imaging spectral in the future.