In order to improve the light energy of spectral imaging system, a visible light AOTF with light aperture of 20 mm×20 mm is presented in this paper. Compared with the common AOTF (light aperture of 10 mm×10 mm), its light flux is increased by three times. In this paper, the diffraction efficiency formula of AOTF including the attenuation coefficient of sound wave, operating frequency and the distance from light to transducer is also derived for the first time. The diffraction efficiency of AOTF can be calculated more accurately by using this foumula. The experimental results show that the AOTF with two transducers has a operating bandwidth of nearly 0.95 octave, filtering range 0.4 μm to 0.9 μm, diffraction efficiency of greater than 60%, spectral resolution of less than 10 nm.