The invention discloses a technique and device for preparing phosphoric acid by a fluidized bed reactor. The technique mainly comprises the following steps: crushing phosphorite, silica and carbon reducer, screening, adding bentonite, adding water, evenly mixing, drying to solidify, recrushing, screening into a granular material, putting the granular material into a fluidized bed reactor, heating to 1100-1350 DEG C by fuel burning and high-temperature gas heat exchange in the oxidizing zone, and fluidizing and reducing under the actions of N2 and reflux technique tail gas; and after the generated P2 and CO enter the oxidizing zone together with external oxygen-enriched air and are burnt to generate P2O5 and CO2 and the P2O5 gas is purified by dust removal, absorbing with recycle acid to obtain H3PO4. The fluidized bed reactor is effectively divided into a reducing zone and an oxidizing zone through a heat conducting plate, and implements heat coupling; and the phosphorite is fluidized, reduced, and subjected to forced heating and mass transfer, thereby greatly lowering the reduction temperature, shortening the reaction time and solving the problems of plugging and ringing in the rotary kiln, and the like.