<p>Fluorine pollution in saline-sodic soils has become an important environmental problem, but little is known about the interaction between aluminium sulfate (Al₂(SO₄)₃) and fluorine when Al₂(SO₄)₃ is used to improve saline-sodic soils. It is vital to clarify the effect of Al₂(SO₄)₃ on fluorine adsorption by saline-sodic soils, which will provide theoretical support for evaluating the risk of fluorine pollution in the improved saline-sodic soils. Batch adsorption experiments were conducted to investigate the impacts of different saline-sodic soils with Al₂(SO₄)₃ on fluorine adsorption, and the mechanisms of adsorption were characterized by scanning electron microscopy, specific surface area and pore size analyzer, Fourier transform infrared spectrometry, and X-ray photoelectron spectroscopy. Fluorine adsorption decreased with increasing soil salinity, and Al₂(SO₄)₃ addition increased the adsorption of fluorine in soils. Among the three saline-sodic soils, Al₂(SO₄)₃ contributed greatly to fluorine adsorption by mildly saline-sodic soils, with a range of 40.84-69.70%. Fluorine adsorption isotherms could be properly described by the Langmuir and Freundlich models. At higher fluorine equilibrium concentrations, adding Al₂(SO₄)₃ first and then adding fluorine, the adsorption amount of fluorine was greater for soils. Both the soil porosity and the numbers of aggregate were increased after adding Al₂(SO₄)₃. Functional groups such as siloxy group, hydrocarbyl group, hydroxyl group and hydroxyaluminum group were involved in the reaction process. Fluorine adsorption on saline-sodic soils refers to the ligand exchange and electrostatic adsorption, and fluorine ion replaces hydroxyl group on the soil surface. This work may provide a theoretical reference for the changes in fluorine chemical behavior when Al₂(SO₄)₃ ameliorates saline-sodic soils.</p>