A preparation method of an epoxy compound having an alkoxysilyl group, the method comprising reacting an epoxy compound having a hydroxyl group with an isocyanate alkoxysilane of Formula 1 below in the presence of at least one amine-based base catalyst selected from the group consisting of triethylamine, diisopropylethylamine, pyridine, and imidazole, wherein the epoxy compound having an alkoxysilyl group has a mole ratio of [epoxide group]:[alkoxysilyl group] of n:1 ranging from 2:1 to 10:1, and is one selected from the group consisting of Formulae AF to IF below: in Formula BF, S is in Formula DF, t is in Formulae AF to DF, n is an integer equal to or greater than 1, in Formula EF, -q- is -CH 2 - or a direct linkage, in Formula GF, R is hydrogen, a hydroxyl group, a C1-C10 alkyl group, or a C6 or C10 aromatic group, in Formulae AF to IF, at least one of Ms is a structure having an alkoxysilyl group selected from the group consisting of -CH 2 CHOXCH 2 OX (Formula S21), -CH 2 CHOXCH 2 Cl (Formula S22), and -CH(CH 2 OX)(CH 2 Cl) (Formula S23), and the remainder are structures having an epoxide group of Formula E1 below: in Formulae S21 to S23, X is CONH(CH 2) 3 SiR 1 R 2 R 3, and at least one of R 1 to R 3 is an alkoxy group having 1 to 5 carbon atoms and the remainder are alkyl groups having 1 to 10 carbon atoms, the structures of Formulae AF to IF may be connected to each other at one of Ms through a linker having an alkoxysilyl group of Formula LG2 below: in Formula LG2, at least one of R 1 to R 3 is an alkoxy group having 1 to 5 carbon atoms, and the remainder are alkyl groups having 1 to 10 carbon atoms; and an increase in an epoxy equivalent weight of the epoxy compound having an alkoxysilyl group is less than 260/n, where n is mole ratio of an epoxide group to 1 mol of the alkoxysilyl group in the epoxy compound having an alkoxysilyl group, and n ranges from 2 to 10, compared to an EEW of the epoxy compound having a hydroxyl group, [Formula 1] OCN(CH 2) 3 SiR 1 R 2 R 3 where at least one of R 1 to R 3 is an alkoxy group having 1 to 5 carbon atoms, and the remainder are alkyl groups having 1 to 10 carbon atoms. wherein the epoxy compound having an alkoxysilyl group has a mole ratio of [epoxide group]:[alkoxysilyl group] of n:1 ranging from 2:1 to 10:1, and is one selected from the group consisting of Formulae AF to IF below: in Formula BF, S is in Formula DF, t is in Formulae AF to DF, n is an integer equal to or greater than 1, in Formula EF, -q- is -CH 2 - or a direct linkage, in Formula GF, R is hydrogen, a hydroxyl group, a C1-C10 alkyl group, or a C6 or C10 aromatic group, in Formulae AF to IF, at least one of Ms is a structure having an alkoxysilyl group selected from the group consisting of -CH 2 CHOXCH 2 OX (Formula S21), -CH 2 CHOXCH 2 Cl (Formula S22), and -CH(CH 2 OX)(CH 2 Cl) (Formula S23), and the remainder are structures having an epoxide group of Formula E1 below: in Formulae S21 to S23, X is CONH(CH 2) 3 SiR 1 R 2 R 3, and at least one of R 1 to R 3 is an alkoxy group having 1 to 5 carbon atoms and the remainder are alkyl groups having 1 to 10 carbon atoms, the structures of Formulae AF to IF may be connected to each other at one of Ms through a linker having an alkoxysilyl group of Formula LG2 below: in Formula LG2, at least one of R 1 to R 3 is an alkoxy group having 1 to 5 carbon atoms, and the remainder are alkyl groups having 1 to 10 carbon atoms; and an increase in an epoxy equivalent weight of the epoxy compound having an alkoxysilyl group is less than 260/n, where n is mole ratio of an epoxide group to 1 mol of the alkoxysilyl group in the epoxy compound having an alkoxysilyl group, and n ranges from 2 to 10, compared to an EEW of the epoxy compound having a hydroxyl group, [Formula 1] OCN(CH 2) 3 SiR 1 R 2 R 3 where at least one of R 1 to R 3 is an alkoxy group having 1 to 5 carbon atoms, and the remainder are alkyl groups having 1 to 10 carbon atoms.