The invention discloses a magnetic arsenic adsorbent as well as a preparation method and application thereof. The preparation method comprises the following step of: reacting in FeCl2 liquor mixed with magnetic nano particles by using NaOH liquor as a pH value regulator and using hydrogen peroxide as an oxidant, so as to obtain the magnetic absorbent. Specially, the preparation method comprises the following steps of: adding pre-prepared magnetic nano particles into the FeCl2 liquor, uniformly mixing under a normal-temperature condition; dropwise adding NaOH aqueous liquor under a high-speed stirring condition, so as to regulate the pH value to 8.0; adding hydrogen peroxide in the case that green precipitates are generated; and regulating the pH value back to 8.0 by using the NaOH aqueous liquor to obtain the adsorbent. The adsorbnet provided by the invention has a series of advantages of large adsorbing capacity, high adsorbing speed, small influences on the absorbing process, good regenerating performance and the like, is easy to recycle, and has important practical application value.