A method for preparing a modification polymerization initiator, the method comprising: reacting a compound represented by the following Formula 1 and a compound represented by the following Formula 2, wherein the reaction is performed in a continuous reactor comprising a first continuous channel and a second continuous channel, and prior to performing the reaction, the compound represented by Formula 1 is injected into the continuous reactor via the first continuous channel, and the compound represented by Formula 2 is injected into the continuous reactor via the second continuous channel: in Formula 1, X is -NR a R b, -OR c, or -SR d, and R a to R d are each independently an alkyl group of 1 to 30 carbon atoms, an alkenyl group of 2 to 30 carbon atoms, an alkynyl group of 2 to 30 carbon atoms, a cycloalkyl group of 3 to 30 carbon atoms, an aryl group of 6 to 30 carbon atoms, a heteroalkyl group of 1 to 30 carbon atoms, a heteroalkenyl group of 2 to 30 carbon atoms, a heteroalkynyl group of 2 to 30 carbon atoms, a heterocycloalkyl group of 2 to 30 carbon atoms, or a heteroaryl group of 3 to 30 carbon atoms, where each of R a to R d is independently unsubstituted or substituted with a substituent comprising one or more heteroatoms selected from N, O, S, Si and F atoms, and R a and R b are optionally connected with each other to form an aliphatic hydrocarbon ring of 5 to 20 carbon atoms, an aromatic hydrocarbon ring of 6 to 20 carbon atoms, or a heterocycle of 3 to 20 carbon atoms, each of which is independently unsubstituted or substituted with an alkyl group of 1 to 30 carbon atoms, [Formula 2] M-R 1 in Formula 2, M is an alkali metal, and R 1 is hydrogen, an alkyl group of 1 to 30 carbon atoms, an alkenyl group of 2 to 30 carbon atoms, an alkynyl group of 2 to 30 carbon atoms, a cycloalkyl group of 5 to 30 carbon atoms, or an aryl group of 6 to 30 carbon atoms. reacting a compound represented by the following Formula 1 and a compound represented by the following Formula 2, wherein the reaction is performed in a continuous reactor comprising a first continuous channel and a second continuous channel, and prior to performing the reaction, the compound represented by Formula 1 is injected into the continuous reactor via the first continuous channel, and the compound represented by Formula 2 is injected into the continuous reactor via the second continuous channel: in Formula 1, X is -NR a R b, -OR c, or -SR d, and R a to R d are each independently an alkyl group of 1 to 30 carbon atoms, an alkenyl group of 2 to 30 carbon atoms, an alkynyl group of 2 to 30 carbon atoms, a cycloalkyl group of 3 to 30 carbon atoms, an aryl group of 6 to 30 carbon atoms, a heteroalkyl group of 1 to 30 carbon atoms, a heteroalkenyl group of 2 to 30 carbon atoms, a heteroalkynyl group of 2 to 30 carbon atoms, a heterocycloalkyl group of 2 to 30 carbon atoms, or a heteroaryl group of 3 to 30 carbon atoms, where each of R a to R d is independently unsubstituted or substituted with a substituent comprising one or more heteroatoms selected from N, O, S, Si and F atoms, and R a and R b are optionally connected with each other to form an aliphatic hydrocarbon ring of 5 to 20 carbon atoms, an aromatic hydrocarbon ring of 6 to 20 carbon atoms, or a heterocycle of 3 to 20 carbon atoms, each of which is independently unsubstituted or substituted with an alkyl group of 1 to 30 carbon atoms, [Formula 2] M-R 1 in Formula 2, M is an alkali metal, and R 1 is hydrogen, an alkyl group of 1 to 30 carbon atoms, an alkenyl group of 2 to 30 carbon atoms, an alkynyl group of 2 to 30 carbon atoms, a cycloalkyl group of 5 to 30 carbon atoms, or an aryl group of 6 to 30 carbon atoms. X is -NR a R b, -OR c, or -SR d, and R a to R d are each independently an alkyl group of 1 to 30 carbon atoms, an alkenyl group of 2 to 30 carbon atoms, an alkynyl group of 2 to 30 carbon atoms, a cycloalkyl group of 3 to 30 carbon atoms, an aryl group of 6 to 30 carbon atoms, a heteroalkyl group of 1 to 30 carbon atoms, a heteroalkenyl group of 2 to 30 carbon atoms, a heteroalkynyl group of 2 to 30 carbon atoms, a heterocycloalkyl group of 2 to 30 carbon atoms, or a heteroaryl group of 3 to 30 carbon atoms, where each of R a to R d is independently unsubstituted or substituted with a substituent comprising one or more heteroatoms selected from N, O, S, Si and F atoms, and R a and R b are optionally connected with each other to form an aliphatic hydrocarbon ring of 5 to 20 carbon atoms, an aromatic hydrocarbon ring of 6 to 20 carbon atoms, or a heterocycle of 3 to 20 carbon atoms, each of which is independently unsubstituted or substituted with an alkyl group of 1 to 30 carbon atoms, [Formula 2] M-R 1 in Formula 2, M is an alkali metal, and R 1 is hydrogen, an alkyl group of 1 to 30 carbon atoms, an alkenyl group of 2 to 30 carbon atoms, an alkynyl group of 2 to 30 carbon atoms, a cycloalkyl group of 5 to 30 carbon atoms, or an aryl group of 6 to 30 carbon atoms. M is an alkali metal, and R 1 is hydrogen, an alkyl group of 1 to 30 carbon atoms, an alkenyl group of 2 to 30 carbon atoms, an alkynyl group of 2 to 30 carbon atoms, a cycloalkyl group of 5 to 30 carbon atoms, or an aryl group of 6 to 30 carbon atoms.