A method for preparing a modified polymerization initiator, the method comprising the steps of: introducing a first fluid through a first fluid inlet and a second fluid through a second fluid inlet into a reactor having the first fluid inlet including a first functional group-containing compound and the second fluid inlet including a second functional group-containing compound, and reacting the first functional group-containing compound with the second functional group-containing compound; and obtaining a third fluid including the modified polymerization initiator prepared by the reaction of the step through an outlet provided in the reactor, wherein the step and step are continuously performed, wherein the modified functional group-containing compound is a compound represented by Formula 1 or a compound according to formula 2 below wherein in Formula 1, R 1 to R 4 may be each independently a monovalent hydrocarbon group having 1 to 30 carbon atoms, a heteroalkyl group having 1 to 30 carbon atoms including at least one heteroatom selected from the group consisting of N, O and S, or a heterocyclic group having 4 to 30 carbon atoms including at least one heteroatom selected from the group consisting of N, O and S; wherein in Formula 2, R 5 to R 7 may be each independently a monovalent hydrocarbon having 1 to 30 carbon atoms, a heteroalkyl group having 1 to 30 carbon atoms and including at least one heteroatom selected from N, O and S, or a heterocyclic group having 4 to 30 carbon atoms and including at least one heteroatom selected from the group consisting of N, O and S; alternatively, R 5 and R 6, R 6 and R 7, or R 5 and R 7 may be bonded to each other to form a hydrocarbon ring group having 5 to 30 carbon atoms, and when R 5 and R 6, R 6 and R 7, or R 5 and R 7 may be bonded to each other to form a hydrocarbon ring group, the hydrocarbon ring may include -CR 8 R 9 -, -NR 10 -, -O-, or -S- in a ring; R 8, R 9, and R 10 may be each independently hydrogen, a monovalent hydrocarbon group having 1 to 30 carbon atoms, a heteroalkyl group having 1 to 30 carbon atoms and including at least one heteroatom selected from the group consisting of N, O and S, or a heterocyclic group having 4 to 30 carbon atoms and including at least one heteroatom selected from the group consisting of N, O and S; wherein the polymerization initiating functional group-containing compound is a compound represented by Formula 3 below wherein in Formula 3, R 11 may be a monovalent hydrocarbon group having 1 to 30 carbon atoms, and M may be a metal, specifically, an alkali metal; wherein the first functional group-containing compound and the second functional group-containing compound are a modified functional group-containing compound and a polymerization-initiating functional group-containing compound, wherein when the first functional group-containing compound is a modified functional group-containing compound, the second functional group-containing compound is a polymerization-initiating functional group-containing compound; and when the first functional group-containing compound is a polymerization-initiating functional group-containing compound, the second functional group-containing compound is a modified functional group-containing compound; characterized in that in the step, the flow amounts measured in g/h of the first fluid and the second fluid are maintained constant at the time when the first fluid and the second fluid are mixed, and the flow rate measured in mm/s of the first fluid is increased at the time when the first fluid and the second fluid are mixed. introducing a first fluid through a first fluid inlet and a second fluid through a second fluid inlet into a reactor having the first fluid inlet including a first functional group-containing compound and the second fluid inlet including a second functional group-containing compound, and reacting the first functional group-containing compound with the second functional group-containing compound; and obtaining a third fluid including the modified polymerization initiator prepared by the reaction of the step through an outlet provided in the reactor,