The present study is focused on the synthesis of scorodite (FeAsO4?2H2O) at 95℃ and initial pH 1.5 under atmospheric pressure. In particular, the effects of initial ferric ion concentration of the solution on the arsenic removal rate and the scorodite formation were investigated in As(V)?Fe(II)?Fe(III) system. The experimental results showed that the well crystalline scorodite was formed through oxidizing ferrous ions by air stream when there was no additional ferric ion in the solution, but only 24.3% of arsenic could be removed and the United States Environmental Protection Agency's Toxicity Characteristic Leaching Procedure tests showed that the arsenic leaching concentration of scorodite was more than 5 mg/L, higher than the arsenic concentration limit set by the national standard. In all the experiments with the additional ferric ions, it was found that there was amorphous ferric arsenate formed in the heating process. The amorphous ferric arsenate was found to converted to crystalline scorodite in 8 h if the initial Fe(III)/As(V) molar ratio was not higher than 1.6. However, the crystallinity of scorodite decreased with the increase of additional ferric ion concentration, the arsenic leaching concentration of solid products decreased and the arsenic removal rate of the solution were improved. The results of the TCLP leaching test showed that at the initial Fe(III)/As(V) molar ratio of 0.8 and 1.6, the arsenic leaching concentration of the formed solid products were less than 5 mg/L, which could meet the emission standards, so that the solid products may be considered to be stable for safe disposal. Nevertheless, if the initial Fe(III)/As(V) molar ratio was higher than 1.6, the amorphous ferric arsenate could not convert to crystalline scorodite after 8 h. The arsenic removal rate decreased and the amorphous ferric arsenate was unstable for high arsenic leaching concentration. Experimental results suggested that adding appropriate ferric in solution containing arsenic and ferrous could contribute to forming stable scorodite and promoting arsenic precipitation.