An ultra-compact and efficient acousto-optic modulator based on a thin-film lithium niobate-chalcogenide (ChG) hybrid platform was designed and realized. In this approach, π phase-shift Bragg grating has an ultra-short effective interaction length of only ∼ 300 µm and a compact footprint of 200 × 300 µm2. The strong microwave-acoustic coupling and superior photo-elastic property of the ChG allow us to achieve a half-wave voltage of Vπ = 1.08 V (4.07 V) for the π phase-shift Bragg grating (waveguide Bragg grating), corresponding to VπL = 0.03 V·cm (0.09 V·cm). This acousto-optic modulator exhibits a compact size, and low power consumption, and can be used for on-chip optical interconnects and microwave photonics.