Abstract: Acid-modified natural magnetite shows excellent arsenic removal properties due to its change in specific surface area and internal structure. It is essential to study further its adsorption to arsenic in water, in order to provide data for practical engineering applications. The feature of arsenic adsorption by modified magnetite immersed in 0.5 mol/L hydrochloric acid and ignited for 10 minutes at 150℃, and the construction of its adsorption rate curve chart is described in this paper. Results of the experiment on adsorption influencing factors indicate that when initial pH value are 6-9, magnetite shows strong adsorption property. Ions such as Cl^-, Ca²⁺, Mg²⁺, HCO₃^-, CO₃^2- do not have competitive adsorption with As(Ⅲ) and As(Ⅴ), but PO₄^3-, NO₃^-, and SO₄^2- have competitive adsorption with As(Ⅲ), in the order of PO₄^3->SO₄^2- >NO₃^-. PO₄^3- and NO₃^- have competitive adsorption with As(Ⅴ). In combination with XRD, SEM, and other research results, preliminarily studies show the arsenic removal mechanism of modified magnetite, and conclude that modified magnetite shows an obviously increased specific surface area and that Fe(Ⅱ) and Fe(Ⅲ) contained in its surface products are the main cause for improving arsenic adsorption ability. Experimental results demonstrate that modified natural magnetite is a de-arsenic material for water, which deserves further study and practical application.