Transfer function of single-stage acousto-optic tunable filter (AOTF) has sidelobes higher than -9 dB, and this large sidelobes level limits AOTF application. In this paper, the sidelobes of two single-stage AOTF's, which are polarization-dependent collinear and polarization-independent quasi-collinear respectively, are investigated, and the transfer function of double-stage filter is obtained. The calculations on single- and double-stage AOTF's have shown that the double-stage device can provide significant improvement in sidelobe suppression. In experiment, two single-stage AOTF's are cascaded, and filtering property of this double-stage device is measured. The experimental results show that the sidelobes of this cascaded AOTF is suppressed to -19dB，and the 3-dB bandwidth is narrowed to some outent.