The invention discloses a suspension liquid-state iron-containing composite 
reagent as well as a preparation method thereof and a method for efficiently removing 
selenate in water and belongs to the technical field of 
water treatment. The method for efficiently removing 
selenate in water comprises the following three steps: step one, preparing the suspension liquid-state iron-containing composite 
reagent from zero-valence 
iron powder, water, 
hydrogen peroxide and 
hydrochloric acid; step two, removing 
selenate by using the suspension liquid-state iron-containing composite 
reagent, and integrally adding the prepared suspension liquid-state iron-containing composite reagent containing a 
solid phase, a colloidal phase and a 
liquid phase into selenate-containing water to be treated; and step three, carrying out 
solid-liquid separation. The suspension liquid-state iron-containing composite reagent has the advantages that the iron-containing composite reagent is simple to prepare and low in cost of raw materials, the iron-containing composite reagent contains a plurality of reduction 
active components, the removal of 
selenic acid through a reduction method is facilitated, and the iron-containing composite reagent is rich in colloidal phase and high in efficiency of transferring 
mass with solution, has both adsorption and 
flocculation effects, is capable of efficiently removing selenate, can still efficiently operate in the co-presence of anions such as high-concentration 
sulfate radicals and the like, is especially applicable to high-salt 
selenium-containing 
wastewater, is environmental-friendly, and is free of secondary 
pollution.