Abstract: In a gold mine,ultra-fine tailings with + 37 μm content below 25% and - 19 μm content above 60% were obtained by cyclone classification of graded fine tailings. Based on the two-stage classified ultra-fine tailings,indoor static flocculation sedimentation test and semi industrial thickening test were carried out. The effects of mortar concentration and flocculant unit consumption on the floc settlement behavior was analyzed by static settlement. It is found that the floc settlement speed decreases with the increase of mortar concentration,while the flocculant unit consumption has an optimal value. If it is higher or lower than this value,the floc settlement speed becomes slower. The influence of the height of the mud layer on the thickening was explored,and the influence of the solid flux on the solid content and underflow concentration of the overflow water was studied by semi industrial thickening test. The experimental results show that the height of the mud layer is nonlinear to the dense underflow concentration,and the underflow concentration increases first and then tends to be stable with the increase of the height of the mud layer. With the increase of solid flux,the solid content of overflow water increases gradually,while the underflow concentration is linearly and negatively correlated with solid flux. The experimental data were successfully applied to 1 000 m³ unpowered high-efficiency thickener to realize industrial application.