The invention provides a magnetic iron oxide/sepiolite nanocomposite. The nanocomposite is powdery and prepared through the following steps that sepiolite and glucose serve as raw materials, mixed liquid A is prepared, a hydrothermal carbonization reaction is conducted, a generated brown substance is washed, and after the brown substance is dried, sepiolite nanofibers with multifunctional active groups are acquired; mixed liquid B is prepared from the sepiolite nanofibers and ferric acetylacetonate and stirred, after being evenly mixed, the sepiolite nanofibers and the ferric acetylacetonate are placed in a reaction kettle for a solvothermal reaction, a generated substance is washed, dried and ground into powder, aerobic calcination is conducted, and therefore the magnetic iron oxide/sepiolite nanocomposite is acquired. By the adoption of the magnetic iron oxide/sepiolite nanocomposite, inorganic trivalent arsenic removal efficiency is high, capacity is large, cyclic use is achieved, the nanocomposite has the magnetic performance so as to be separated rapidly, and recycling and reuse are achieved. Meanwhile, a preparation method of the magnetic iron oxide/sepiolite nanocomposite is provided and the magnetic iron oxide/sepiolite nanocomposite has the advantages of being low in cost, easy and convenient to operate, easy to synthesize on a large scale and the like.