The invention provides a method combining an oxidizing composite reagent and activated carbon to remove arsenic in water. The oxidizing composite reagent is added into the water and stirred, and then the activated carbon is utilized to adsorb the arsenic. The oxidizing composite reagent is formed by compositing potassium ferrate, chlorine dioxide, sodium peroxide, potassium persulfate, potassium monopersulfate, ferrous sulfate, ferric sulfate and hydroxylamine hydrochloride, or is formed by compositing potassium peroxide, sodium peroxide, sodium monopersulfate, iron chloride and hydroxylaminehydrochloride, or is formed by compositing calcium peroxide, sodium hypochlorite, calcium monopersulfate, calcium persulfate, cerium chloride, ferric chloride, cerous sulfate and humus, or is formed by compositing cobalt chloride, ammonium monopersulfate, ammonium persulfate, cerous sulfate, sodium peroxide and hydroxylamine hydrochloride. The method can not only remove the arsenic in drinking water, underground water and surface water rapidly, efficiently, conveniently and safely, but also effectively reduce concentration of the arsenic in sewage containing arsenic and effluent of sewage secondary sedimentation tanks, and simultaneously can have good recovery effect on lakes and inland seawater polluted by the arsenic.