Flotation tests, contact angle measurements, infrared spectrum analyses, X-ray analyses and computer simulation were carried out in order to study the activation mechanism of lime-depressed pyrite with oxalic acid. The results show that the oxalic acid effectively eliminated the hydrophilic calcium film from the surface of pyrite. Therefore, the efficiency of pyrite flotation was also activated. The results indicate that after reacting with hydrophobic insoluble remainders on the surface of pyrite, oxalic acid can produce hydrophilic compounds, such as CaCO 3 , Ca(OH) 2 and Fe(OH) 3 . As a consequence, a fresh pyrite layer was exposed and its flotation activated. Flotation tests, contact angle measurements, infrared spectrum analyzes, X-ray analyzes and computer simulation were carried out in order to study the activation mechanism of lime-depressed pyrite with oxalic acid. The results show that the oxalic acid effectively eliminated the hydrophilic calcium film from the surface of pyrite. Thus, the efficiency of pyrite flotation was also activated. The results that that reacting with hydrophobic insoluble remainders on the surface of pyrite, oxalic acid can produce hydrophilic compounds, such as CaCO 3, Ca (OH) 2 and Fe (OH) 3. As a consequence, a fresh pyrite layer was exposed and its flotation activated.