A method of preparing a polyolefin, the method comprising the step of polymerizing olefin monomers in the presence of a metallocene catalyst and a molecular weight modifier, and in a cascade reactor, wherein the molecular weight modifier is represented by the following Chemical Formula 5, Chemical Formula 6, or Chemical Formula 7: and wherein the metallocene catalyst includes one of more metallocene compounds represented by any one of the following Chemical Formulae 9 and 12: [Chemical Formula 9] (Cp 5 R a) n (Cp 6 R b)M 3 Z 1 3-n wherein M 3 is a Group 4 transition metal; Cp 5 and Cp 6 are the same as or different from each other, and are each independently any one selected from the group consisting of cyclopentadienyl, indenyl, 4,5,6,7-tetrahydro-1-indenyl, and fluorenyl radicals, and these are substituted with a hydrocarbon having 1 to 20 carbon atoms; R a and R b are the same as or different from each other, and are each independently hydrogen, a C1 to C20 alkyl, a C1 to C10 alkoxy, a C2 to C20 alkoxyalkyl, a C6 to C20 aryl, a C6 to C10 aryloxy, a C2 to C20 alkenyl, a C7 to C40 alkylaryl, a C7 to C40 arylalkyl, a C8 to C40 arylalkenyl, or a C2 to C10 alkynyl; Z 1 is a halogen atom, a C1 to C20 alkyl, a C2 to C10 alkenyl, a C7 to C40 alkylaryl, a C7 to C40 arylalkyl, a C6 to C20 aryl, a substituted or unsubstituted C1 to C20 alkylidene, a substituted or unsubstituted amino group, a C2 to C20 alkylalkoxy, or a C7 to C40 arylalkoxy; and n is 1 or 0; wherein A is hydrogen, a halogen, a C1 to C20 alkyl, a C2 to C20 alkenyl, a C6 to C20 aryl, a C7 to C20 alkylaryl, a C7 to C20 arylalkyl, a C1 to C20 alkoxy, a C2 to C20 alkoxyalkyl, a C3 to C20 heterocycloalkyl, or a C5 to C20 heteroaryl; D is -O-, -S-, -N -, or -Si (R')-, in which R and R' are the same as or different from each other, and are each independently hydrogen, a halogen, a C1 to C20 alkyl, a C2 to C20 alkenyl, or a C6 to C20 aryl; L is a C1 to C10 straight or branched alkylene; B is carbon, silicon, or germanium; Q is hydrogen, halogen, C1 to C20 alkyl, C2 to C20 alkenyl, C6 to C20 aryl, C7 to C20 alkylaryl, or C7 to C20 arylalkyl; M is a Group 4 transition metal; X 1 and X 2 are the same as or different from each other, and are each independently a halogen, a C1 to C20 alkyl, a C2 to C20 alkenyl, a C6 to C20 aryl, a nitro group, an amido group, a C1 to C20 alkylsilyl, a C1 to C20 alkoxy, or a C1 to C20 sulfonate; C 1 and C 2 are the same as or different from each other, and are each independently represented by any one of the following Chemical Formula 13a, Chemical Formula 13b, and Chemical Formula 13c, excluding that both C 1 and C 2 are Chemical Formula 13c; wherein R1 to R17 and R1' to R9' are the same as or different from each other, and are each independently hydrogen, a halogen, a C1 to C20 alkyl, a C2 to C20 alkenyl, a C1 to C20 alkylsilyl, a C1 to C20 silylalkyl, a C1 to C20 alkoxysilyl, a C1 to C20 alkoxy, a C6 to C20 aryl, a C7 to C20 alkylaryl, or a C7 to C20 arylalkyl, and two or more neighboring groups of R10 to R17 are connected to each other to form a substituted or unsubstituted aliphatic or aromatic ring. wherein M 3 is a Group 4 transition metal; Cp 5 and Cp 6 are the same as or different from each other, and are each independently any one selected from the group consisting of cyclopentadienyl, indenyl, 4,5,6,7-tetrahydro-1-indenyl, and fluorenyl radicals, and these are substituted with a hydrocarbon having 1 to 20 carbon atoms; R a and R b are the same as or different from each other, and are each independently hydrogen, a C1 to C20 alkyl, a C1 to C10 alkoxy, a C2 to C20 alkoxyalkyl, a C6 to C20 aryl, a C6 to C10 aryloxy, a C2 to C20 alkenyl, a C7 to C40 alkylaryl, a C7 to C40 arylalkyl, a C8 to C40 arylalkenyl, or a C2 to C10 alkynyl; Z 1 is a halogen atom, a C1 to C20 alkyl, a C2 to C10 alkenyl, a C7 to C40 alkylaryl, a C7 to C40 arylalkyl, a C6 to C20 aryl, a substituted or unsubstituted C1 to C20 alkylidene, a substituted or unsubstituted amino group, a C2 to C20 alkylalkoxy, or a C7 to C40 arylalkoxy; and n is 1 or 0; wherein A is hydrogen, a halogen, a C1 to C20 alkyl, a C2 to C20 alkenyl, a C6 to C20 aryl, a C7 to C20 alkylaryl, a C7 to C20 arylalkyl, a C1 to C20 alkoxy, a C2 to C20 alkoxyalkyl, a C3 to C20 heterocycloalkyl, or a C5 to C20 heteroaryl; D is -O-, -S-, -N -, or -Si (R')-, in which R and R' are the same as or different from each other, and are each independently hydrogen, a halogen, a C1 to C20 alkyl, a C2 to C20 alkenyl, or a C6 to C20 aryl; L is a C1 to C10 straight or branched alkylene; B is carbon, silicon, or germanium; Q is hydrogen, halogen, C1 to C20 alkyl, C2 to C20 alkenyl, C6 to C20 aryl, C7 to C20 alkylaryl, or C7 to C20 arylalkyl; M is a Group 4 transition metal; X 1 and X 2 are the same as or different from each other, and are each independently a halogen, a C1 to C20 alkyl, a C2 to C20 alkenyl, a C6 to C20 aryl, a nitro group, an amido group, a C1 to C20 alkylsilyl, a C1 to C20 alkoxy, or a C1 to C20 sulfonate; C 1 and C 2 are the same as or different from each other, and are each independently represented by any one of the following Chemical Formula 13a, Chemical Formula 13b, and Chemical Formula 13c, excluding that both C 1 and C 2 are Chemical Formula 13c;