A method for preparing a long fiber-reinforcing olefin polymer, the method comprising the steps of: polymerizing olefin monomers in the presence of a supported catalyst in which a transition metal compound represented by the following Chemical Formula 1, a cocatalyst represented by the following Chemical Formula 2, and a cocatalyst represented by the following Chemical Formula 3 are supported on a carrier, wherein an olefin polymer having a degree of stereoregularity of 95% by weight or more is provided, the degree of stereoregularity being measured according to the method defined in the description; in Chemical Formula 1, M is one of a Group 3 transition metal, a Group 4 transition metal, a Group 5 transition metal, a lanthanide series transition metal, and an actinide series transition metal, X 1 and X 2 are the same as or different from each other, and are each independently one of halogen, A is one of Group 14 elements, n is an integer between 1 and 20, R 1 is one of an alkyl having 1 to 20 carbon atoms, an alkenyl having 2 to 20 carbon atoms, an alkylaryl having 7 to 30 carbon atoms, an arylalkyl having 7 to 30 carbon atoms, and an aryl having 6 to 30 carbon atoms, R 2 is one of hydrogen, an alkyl having 1 to 20 carbon atoms, an alkenyl having 2 to 20 carbon atoms, an alkylaryl having 7 to 30 carbon atoms, an arylalkyl having 7 to 30 carbon atoms, and an aryl having 6 to 30 carbon atoms, and R 3 to R 6 are each independently one of an alkyl having 1 to 20 carbon atoms, [Chemical Formula 2] R 8 -[Al(R 7)-O] m -R 9 in Chemical Formula 2, R 7, R 8 and R 9 are each independently one of hydrogen, halogen, a hydrocarbyl group having 1 to 20 carbon atoms and a hydrocarbyl group having 1 to 20 carbon atoms substituted with halogen, and m is an integer of 2 or more, [Chemical Formula 3] [L-H] + [W 4] - or + [W 4] - wherein, L is a neutral or cationic Lewis base, W is a Group 13 element, J is each independently a hydrocarbyl group having 1 to 20 carbon atoms; a hydrocarbyloxy group having 1 to 20 carbon atoms; and substituents in which at least one hydrogen atom of the above substituent groups is substituted with at least one substituent selected from halogen, a hydrocarbyloxy group having 1 to 20 carbon atoms and a hydrocarbyl(oxy)silyl group having 1 to 20 carbon atoms. M is one of a Group 3 transition metal, a Group 4 transition metal, a Group 5 transition metal, a lanthanide series transition metal, and an actinide series transition metal, X 1 and X 2 are the same as or different from each other, and are each independently one of halogen, A is one of Group 14 elements, n is an integer between 1 and 20, R 1 is one of an alkyl having 1 to 20 carbon atoms, an alkenyl having 2 to 20 carbon atoms, an alkylaryl having 7 to 30 carbon atoms, an arylalkyl having 7 to 30 carbon atoms, and an aryl having 6 to 30 carbon atoms, R 2 is one of hydrogen, an alkyl having 1 to 20 carbon atoms, an alkenyl having 2 to 20 carbon atoms, an alkylaryl having 7 to 30 carbon atoms, an arylalkyl having 7 to 30 carbon atoms, and an aryl having 6 to 30 carbon atoms, and R 3 to R 6 are each independently one of an alkyl having 1 to 20 carbon atoms, [Chemical Formula 2] R 8 -[Al(R 7)-O] m -R 9 in Chemical Formula 2, R 7, R 8 and R 9 are each independently one of hydrogen, halogen, a hydrocarbyl group having 1 to 20 carbon atoms and a hydrocarbyl group having 1 to 20 carbon atoms substituted with halogen, and m is an integer of 2 or more, [Chemical Formula 3] [L-H] + [W 4] - or + [W 4] - wherein, L is a neutral or cationic Lewis base, W is a Group 13 element, J is each independently a hydrocarbyl group having 1 to 20 carbon atoms; a hydrocarbyloxy group having 1 to 20 carbon atoms; and substituents in which at least one hydrogen atom of the above substituent groups is substituted with at least one substituent selected from halogen, a hydrocarbyloxy group having 1 to 20 carbon atoms and a hydrocarbyl(oxy)silyl group having 1 to 20 carbon atoms. R 7, R 8 and R 9 are each independently one of hydrogen, halogen, a hydrocarbyl group having 1 to 20 carbon atoms and a hydrocarbyl group having 1 to 20 carbon atoms substituted with halogen, and m is an integer of 2 or more, [Chemical Formula 3] [L-H] + [W 4] - or + [W 4] - wherein, L is a neutral or cationic Lewis base, W is a Group 13 element, J is each independently a hydrocarbyl group having 1 to 20 carbon atoms; a hydrocarbyloxy group having 1 to 20 carbon atoms; and substituents in which at least one hydrogen atom of the above substituent groups is substituted with at least one substituent selected from halogen, a hydrocarbyloxy group having 1 to 20 carbon atoms and a hydrocarbyl(oxy)silyl group having 1 to 20 carbon atoms.