A method for preparing a butene oligomer, the butene oligomer produced by the oligomerization having a number average molecular weight of 500 to 5,500 g/mol, the method comprising: a step of oligomerizing a polymerization solution comprising a halogenated hydrocarbon solvent, a nonpolar hydrocarbon solvent and an isobutene monomer, in the presence of an organometal catalyst represented by the following Formula 1: in Formula 1, M is one or more selected from the group consisting of Al, Ga, In, Tl, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, L is one or more selected from the group consisting of acetonitrile, propionitrile, 2-methylpropanenitrile, trimethylacetonitrile, benzonitrile, dialkyl ether, pyridine, dimethylformamide, dimethyl sulfoxide, nitromethane, nitrobenzene and the derivatives thereof, and a coordinating solvent molecule in which an unshared electron pair of oxygen, nitrogen or carbon makes a coordination bond with M, R 1 to R 4 are each independently hydrogen, a halogen group, or a halogen-substituted C1-C12 alkyl group, R 5 and R 6 are each independently hydrogen, a C1-C12 alkyl group, a C6-C12 aryl group, or an allyl group, a, b, c and a+b+c are each independently an integer of 0 to 3, d and a+b+c+d are each independently an integer of 1 to 10, o, p, q and r are each independently an integer of 1 to 5, and x and y are an integer of 1 to 4 and are the same, and a step of removing the organometal catalyst by filtering a oligomerized product, wherein a step of washing the oligomerized product to remove the organometal catalyst is not performed, wherein the organometal catalyst does not comprise a halogen salt of one or more metals selected from the group consisting of metals of group 1, group 2 and group 11, and wherein the organometal catalyst is prepared by not using a metal reagent but using an organo borate-based reagent, and by a preparation method including a step of preparing a dispersion including a metal precursor represented by the following Formula 2 and a coordinating solvent; and a step of reacting an organic borate-based compound including a carbon-based, silyl-based or amine-based cation and a borate-based bulky anion, with the dispersion: [Formula 2] M(OCOR 5) e (NO 3) f (OR 6) g h in Formula 2, M is one or more selected from the group consisting of Al, Ga, In, Tl, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and R 5 and R 6 are each independently hydrogen, a C1-C12 alkyl group, a C6-C12 aryl group, or an allyl group, e, f, g and h are each independently an integer of 0 to 3, and e+f+g+h is 3. a step of oligomerizing a polymerization solution comprising a halogenated hydrocarbon solvent, a nonpolar hydrocarbon solvent and an isobutene monomer, in the presence of an organometal catalyst represented by the following Formula 1: in Formula 1, M is one or more selected from the group consisting of Al, Ga, In, Tl, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, L is one or more selected from the group consisting of acetonitrile, propionitrile, 2-methylpropanenitrile, trimethylacetonitrile, benzonitrile, dialkyl ether, pyridine, dimethylformamide, dimethyl sulfoxide, nitromethane, nitrobenzene and the derivatives thereof, and a coordinating solvent molecule in which an unshared electron pair of oxygen, nitrogen or carbon makes a coordination bond with M, R 1 to R 4 are each independently hydrogen, a halogen group, or a halogen-substituted C1-C12 alkyl group, R 5 and R 6 are each independently hydrogen, a C1-C12 alkyl group, a C6-C12 aryl group, or an allyl group, a, b, c and a+b+c are each independently an integer of 0 to 3, d and a+b+c+d are each independently an integer of 1 to 10, o, p, q and r are each independently an integer of 1 to 5, and x and y are an integer of 1 to 4 and are the same, and a step of removing the organometal catalyst by filtering a oligomerized product, wherein a step of washing the oligomerized product to remove the organometal catalyst is not performed, wherein the organometal catalyst does not comprise a halogen salt of one or more metals selected from the group consisting of metals of group 1, group 2 and group 11, and wherein the organometal catalyst is prepared by not using a metal reagent but using an organo borate-based reagent, and by a preparation method including a step of preparing a dispersion including a metal precursor represented by the following Formula 2 and a coordinating solvent; and a step of reacting an organic borate-based compound including a carbon-based, silyl-based or amine-based cation and a borate-based bulky anion, with the dispersion: [Formula 2] M(OCOR 5) e (NO 3) f (OR 6) g h in Formula 2, M is one or more selected from the group consisting of Al, Ga, In, Tl, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and R 5 and R 6 are each independently hydrogen, a C1-C12 alkyl group, a C6-C12 aryl group, or an allyl group, e, f, g and h are each independently an integer of 0 to 3, and e+f+g+h is 3.