A method for preparing a liquid random copolymer of ethylene and alpha-olefin, comprising a step of: carrying out solution polymerization of ethylene and alpha-olefin having 3 to 20 carbon atoms, under a catalyst system containing a bridged metallocene compound represented by following Formula 1 and at least one compound which reacts with the bridged metallocene compound to form ion pair, wherein R 1, R 2, R 3, R 4, R 5, R 8, R 9 and R 12 are independently hydrogen atom, hydrocarbon group or silicon-containing hydrocarbon group, and adjacent groups can be connected to form a ring structure, R 2 or R 3 bonded to 3-position of cyclopentadienyl group is hydrocarbon group having 4 or more carbon atoms, R 6 and R 11, being the same to each other, are hydrogen atom, hydrocarbon group or silicon-containing hydrocarbon group, R 7 and R 10, being the same to each other, are hydrogen atom, hydrocarbon group or silicon-containing hydrocarbon group, R 6 and R 7 can be connected to hydrocarbon having 2 to 3 carbon atoms to form a ring structure, R 11 and R 10 can be connected to hydrocarbon having 2 to 3 carbon atoms to form a ring structure, R 6, R 7, R 10 and R 11 are not hydrogen atom at the same time; Y is carbon atom or silicon atom; R 13 and R 14 are independently aryl group; M is Ti, Zr or Hf; Q is independently halogen, hydrocarbon group, anionic ligands or a neutral ligand which can be coordinated to a lone pair of electrons; j is an integer of 1~4, and wherein the liquid random copolymer comprises 40 to 60 mol% of ethylene units and 60 to 40 mol% of alpha-olefin unit having 3 to 20 carbon atoms, has number average molecular weight (Mn) of 500 to 10,000 g/mol and a molecular weight distribution (Mw/Mn, Mw is the weight average molecular weight) of 3 or less measured by Gel Permeation Chromatography has Kinematic Viscosity at 100°C of 30 to 5,000 cSt measured according to ASTM D 445, has pour point of 30 to -45°c measured according to ASTM D 6749, and has Bromine Number of 0.1 or less measured in accordance to ASTM D1559. a bridged metallocene compound represented by following Formula 1 and at least one compound which reacts with the bridged metallocene compound to form ion pair, wherein R 1, R 2, R 3, R 4, R 5, R 8, R 9 and R 12 are independently hydrogen atom, hydrocarbon group or silicon-containing hydrocarbon group, and adjacent groups can be connected to form a ring structure, R 2 or R 3 bonded to 3-position of cyclopentadienyl group is hydrocarbon group having 4 or more carbon atoms, R 6 and R 11, being the same to each other, are hydrogen atom, hydrocarbon group or silicon-containing hydrocarbon group, R 7 and R 10, being the same to each other, are hydrogen atom, hydrocarbon group or silicon-containing hydrocarbon group, R 6 and R 7 can be connected to hydrocarbon having 2 to 3 carbon atoms to form a ring structure, R 11 and R 10 can be connected to hydrocarbon having 2 to 3 carbon atoms to form a ring structure, R 6, R 7, R 10 and R 11 are not hydrogen atom at the same time; Y is carbon atom or silicon atom; R 13 and R 14 are independently aryl group; M is Ti, Zr or Hf; Q is independently halogen, hydrocarbon group, anionic ligands or a neutral ligand which can be coordinated to a lone pair of electrons; j is an integer of 1~4, and wherein the liquid random copolymer comprises 40 to 60 mol% of ethylene units and 60 to 40 mol% of alpha-olefin unit having 3 to 20 carbon atoms, has number average molecular weight (Mn) of 500 to 10,000 g/mol and a molecular weight distribution (Mw/Mn, Mw is the weight average molecular weight) of 3 or less measured by Gel Permeation Chromatography has Kinematic Viscosity at 100°C of 30 to 5,000 cSt measured according to ASTM D 445, has pour point of 30 to -45°c measured according to ASTM D 6749, and has Bromine Number of 0.1 or less measured in accordance to ASTM D1559. wherein R 1, R 2, R 3, R 4, R 5, R 8, R 9 and R 12 are independently hydrogen atom, hydrocarbon group or silicon-containing hydrocarbon group, and adjacent groups can be connected to form a ring structure, R 2 or R 3 bonded to 3-position of cyclopentadienyl group is hydrocarbon group having 4 or more carbon atoms, R 6 and R 11, being the same to each other, are hydrogen atom, hydrocarbon group or silicon-containing hydrocarbon group, R 7 and R 10, being the same to each other, are hydrogen atom, hydrocarbon group or silicon-containing hydrocarbon group, R 6 and R 7 can be connected to hydrocarbon having 2 to 3 carbon atoms to form a ring structure, R 11 and R 10 can be connected to hydrocarbon having 2 to 3 carbon atoms to form a ring structure, R 6, R 7, R 10 and R 11 are not hydrogen atom at the same time; Y is carbon atom or silicon atom; R 13 and R 14 are independently aryl group; M is Ti, Zr or Hf; Q is independently halogen, hydrocarbon group, anionic ligands or a neutral ligand which can be coordinated to a lone pair of electrons; j is an integer of 1~4, and wherein the liquid random copolymer comprises 40 to 60 mol% of ethylene units and 60 to 40 mol% of alpha-olefin unit having 3 to 20 carbon atoms, has number average molecular weight (Mn) of 500 to 10,000 g/mol and a molecular weight distribution (Mw/Mn, Mw is the weight average molecular weight) of 3 or less measured by Gel Permeation Chromatography has Kinematic Viscosity at 100°C of 30 to 5,000 cSt measured according to ASTM D 445, has pour point of 30 to -45°c measured according to ASTM D 6749, and has Bromine Number of 0.1 or less measured in accordance to ASTM D1559.