A method for oligomerizing olefin, the method comprising: performing a oligomerization reaction of olefin in the presence of an oligomerization catalyst system; injecting a deactivator in a latter part of a multimerization reaction of olefin to deteriorate activity of the oligomerization catalyst system; and separating an alpha-olefin product and a polymer resin product after deteriorating the activity of the oligomerization catalyst system, wherein the polymer resin product comprises polyethylene and the deactivator, wherein the deactivator comprises an additive for polymer including at least one functional group selected from the group consisting of a hydroxyl group, an amine group and an amide group, wherein the additive for polymer comprises at least one selected from the group consisting of an antistatic agent, an antioxidant, a lubricant, a stabilizer, a light stabilizer and a phase transfer catalyst, wherein the method does not include separating the deactivator from the polymer resin product, wherein the antistatic agent comprises at least one selected from the group consisting of bis(2-hydroxyethyl)pentadecylamine, an ethoxylated fatty amine having 12 to 18 carbon atoms, glycerol monostearate, erucamide, stearamide, oleamide and benenamide, wherein the antioxidant comprises a phenol-based antioxidant, the phenol-based antioxidant comprising butylated hydroxytoluene, wherein the lubricant comprises at least one selected from the group consisting of erucamide, stearamide, oleamide, benenamide, an ethoxylated fatty amine having 12 to 18 carbon atoms and glycerol monostearate, wherein the stabilizer comprises triisopropanolamine, quadrol(N,N,N,N-tetrakis(2-hydroxypropyl)-ethylenediamine) or a mixture thereof, wherein the light stabilizer comprises a benzophenone-based compound, a benzotriazole-based compound or a mixture thereof, and wherein the phase transfer catalyst comprises tricaprylmethylammonium chloride. performing a oligomerization reaction of olefin in the presence of an oligomerization catalyst system; injecting a deactivator in a latter part of a multimerization reaction of olefin to deteriorate activity of the oligomerization catalyst system; and separating an alpha-olefin product and a polymer resin product after deteriorating the activity of the oligomerization catalyst system, wherein the polymer resin product comprises polyethylene and the deactivator, wherein the deactivator comprises an additive for polymer including at least one functional group selected from the group consisting of a hydroxyl group, an amine group and an amide group, wherein the additive for polymer comprises at least one selected from the group consisting of an antistatic agent, an antioxidant, a lubricant, a stabilizer, a light stabilizer and a phase transfer catalyst, wherein the method does not include separating the deactivator from the polymer resin product, wherein the antistatic agent comprises at least one selected from the group consisting of bis(2-hydroxyethyl)pentadecylamine, an ethoxylated fatty amine having 12 to 18 carbon atoms, glycerol monostearate, erucamide, stearamide, oleamide and benenamide, wherein the antioxidant comprises a phenol-based antioxidant, the phenol-based antioxidant comprising butylated hydroxytoluene, wherein the lubricant comprises at least one selected from the group consisting of erucamide, stearamide, oleamide, benenamide, an ethoxylated fatty amine having 12 to 18 carbon atoms and glycerol monostearate, wherein the stabilizer comprises triisopropanolamine, quadrol(N,N,N,N-tetrakis(2-hydroxypropyl)-ethylenediamine) or a mixture thereof, wherein the light stabilizer comprises a benzophenone-based compound, a benzotriazole-based compound or a mixture thereof, and wherein the phase transfer catalyst comprises tricaprylmethylammonium chloride.