The present invention relates to a step-by-step deep purification method for low-concentration arsenic-containing wastewater. For low-concentration arsenic-containing wastewater, a step-by-step treatment method of primary arsenic removal and then deep arsenic removal is adopted, including the following process steps and conditions: primary arsenic removal, initially according to iron ( The mass ratio of aluminum) / arsenic is controlled at 4~10, and coagulant is added, and the pH value of the low-concentration arsenic-containing wastewater in the stirred reaction tank is adjusted to 7~9 with sulfuric acid or milk of lime, and the mass ratio of iron / arsenic is controlled at 4~9 in the follow-up. 10. The reaction time is 5~30min, the stirring line speed is 1.0~2.0m / s, after the coagulation and arsenic removal reaction, the solid-liquid separation is carried out to obtain the primary arsenic removal underflow and the primary arsenic removal overflow; for the deep arsenic removal, the primary arsenic removal The overflow is pumped to the reaction tank through the slurry, and the iron salt is added first, and the iron / arsenic mass ratio is controlled at 9~15, and the reaction is 5~20min, and then the polymer is added at a concentration of 0.1~0.3% and an amount of 3~10g / t The flocculant undergoes coagulation aeration and solid-liquid separation to obtain deep arsenic removal bottom flow and deep arsenic removal overflow, and the deep arsenic removal overflow reaches the standard and is discharged. After the method of the present invention, the concentration of arsenic in the aqueous solution is as low as 0.005 mg / L or less , has the advantages of less chemical consumption, less slag, and high efficiency.