The flotation separation of magnetite and quartz is a long-term challenge for the beneficiation industry. For high-quartz magnetite, conventional flotation shows poor separation effect, resulting in the waste of resources and low flotation efficiency. In this paper, dodecylamine acts as a collector and Al(III) ions in water act as a depressant to selectively separate magnetite and quartz at high alkalinity. The experimental results are analyzed by a micro-flotation experiment, solution chemical calculation, zeta potential, contact angle measurement, and Fourier transform infrared spectroscopy (FTIR). The results of micro-flotation experiments showed that Al(III) ions in water inhibited magnetite more strongly than quartz. The calculation results of solution stoichiometry and zeta potential showed that the phase formed by Al(III) ions on the surface of magnetite and quartz are mainly Al(OH)_3(s), which covers the surface of magnetite and quartz, The contact angle measurement results showed that with the addition of Al(III) ions, the contact angle of magnetite varies significantly than that of quartz, and the floatability of magnetite is lower than that of quartz. The FT-IR results further indicated that the addition of Al(III) ions could hinder the adsorption of dodecylamine on the magnetite surface. Meanwhile, the addition of Al(III) ions has no obvious effect on the adsorption of dodecylamine on the quartz surface.