The action mechanism of over proof ions (NH4+, Mg2+, Ca2+, F-) in industry backwater to flotation of pyrite was investigated by pure mineral flotation tests, ultraviolet visual (UV-Vis) spectrophotometer, Zeta potential and Fourier transform infrared spectroscopy (FT-IR) spectra measurements. The results showed that NH4+ in backwater made the potential of mineral surface increase and extended the range of pH values of pyrite flotation. FT-IR spectra results showed that NH4+ also made xanthate chemisorb on pyrite surface, and adsorption quantity increased with NH4+concentration increasing. The flotation recovery rate increased by 15.5% after activation of NH4+. While Mg2+, Ca2+ and F- in backwater inhibited the adsorption of xanthate on pyrite surface, the flotation recovery rate decreased by 10% on average. Mg2+ and Ca2+ easily formed hydrophilic membranes on pyrite surface to hinder xanthate adsorption; F- adsorption on mineral surface could make mineral surface be negatively charged, which was found by Zeta potential test, and thus formed a competitive adsorption between xanthate ion and F-, both being anions, and reduced xanthate adsorption on pyrite surface, so floatability of pyrite decreased. © Editorial Office of Chinese Journal of Rare Metals. All right reserved.