The invention discloses a remediation agent for treating arsenic contaminated soil and a usage of the remediation agent. The remediation agent comprises the following raw materials by mass: 5-30% of oxidant, 30-70% of iron-based compound and 10-60% of mineral material. The raw materials are respectively ground into materials with particle size not smaller than 200 meshes and are mixed uniformly according to the proportion. The usage of the remediation agent for treating the arsenic contaminated soil comprises the following steps that: the arsenic content and the leaching toxicity of the to-be-treated contaminated soil are detected; the remediation agent with different ratios according to the degree of contamination is paved on the surface of the to-be-treated soil; the soil is turned over and uniformly mixed to form the mixed soil; the water content in the mixed soil is kept not to be less than 25% by adding water; a moisturizing material is covered on the mixed soil; and the mixed soil is cured for at least 5 days in order to allow the remediation agent in the mixed soil to be reacted with unstable arsenic in the heavy metal contaminated soil; and the generated insoluble arsenate sediment is stabilized, so that the cured soil can meet the environment protection requirements.