The present study was directed towards the feasibility and long-term operation efficiency of the purification of groundwater with low concentration of As(Ⅲ), using iron-based adsorbent which was in-situ formed in the filtration process by dosing ferrous sulfate, without pre-oxidation technique. Several experiments were conducted to analyze the optimal dosage ratio of the Fe and As, and the effect of filtration rate and the arsenic removal mechanism in the filtration process. The results showed that with the influent As (Ⅲ) concentrations of 50 and 100 μg/L respectively for the filter column R1 and R2, and the filtration rate of 5 m/h, the optimal ferrous sulfate dosage was 1.2 and 2 mg/L respectively, and the optimal dosage ratio of Fe and As was about 20:1. The filter column R3 was adopted for advanced research, with the influent As (Ⅲ) concentration of 50-70 μg/L and the total Fe concentration of about 2 mg/L. The As(Ⅲ) removal space in the filter column descended continuously with the increasing of filtration rate, and the removal process mainly occured in the upper 60 cm filter layer. Meanwhile, the Fe removal space basically remained unchanged: the removal concentration was both about 1 mg/L for the filter layer of 0-20 cm and 20-80 cm respectively. In the increase of the the filtration rate from 3 m/h to 10 m/h, the maximum filtration rate of the filter column was confirmed to be 10 m/h, and the backwashing period decreased slightly(remaining within 72 h). Intermediate products might be generated in the autocatalytic oxidation process of ferrous ion, which were of great benefit to the oxidation of As (Ⅲ). The r-FeOOH coated on the filter media surface and the Fe(OH)₃ formed in the filter media pore could provide sufficient sites for arsenic adsorption.