A method for preparing a modified and conjugated diene-based polymer, the method comprising: polymerizing conjugated diene-based monomers, or a conjugated diene-based monomer and an aromatic vinyl-based monomer in the presence of an organometal compound in a hydrocarbon solvent to prepare a conjugated diene-based polymer of which at least one terminal comprises an activated organometal part; reacting the conjugated diene-based polymer with an alkoxysilane-based modifier to prepare a modified and conjugated diene-based polymer of which at least one terminal is combined with a functional group derived from the modifier; adding an alcohol and an antioxidant to the modified and conjugated diene-based polymer to deactivate the activated organometal part remaining in the modified and conjugated diene-based polymer to prepare a modified and conjugated diene-based polymer which is deactivated; and adding a modification stabilizer of the following Formula 1 to the modified and conjugated diene-based polymer which is deactivated, to perform a condensation reaction of the functional group derived from a modifier in the modified and conjugated diene-based polymer and the modification stabilizer of the following Formula 1: in Formula 1, m is an integer of 0 to 3, R 1 is a functional group having a structure of the following Formula 1-1, and R 2 and R 3 are each independently one of a halogen group, an alkyl group of 1 to 5 carbon atoms, and an alkoxy group of 1 to 5 carbon atoms, where at least one of R 2 and R 3 is the alkoxy group: [Formula 1-1] [R 11 -(R 12) n]- in Formula 1-1, n is 0 or 1, R 11 is an amino group or a four-member to six-member heterocyclic group containing 1 to 3 nitrogen atoms, which is unsubstituted or substituted with at least one substituent selected from the group consisting of an alkyl group of 1 to 10 carbon atoms and a cycloalkyl group of 3 to 10 carbon atoms, and R 12 is an alkylene group of 4 to 10 carbon atoms, which is unsubstituted or substituted with an alkyl group of 1 to 4 carbon atoms. polymerizing conjugated diene-based monomers, or a conjugated diene-based monomer and an aromatic vinyl-based monomer in the presence of an organometal compound in a hydrocarbon solvent to prepare a conjugated diene-based polymer of which at least one terminal comprises an activated organometal part; reacting the conjugated diene-based polymer with an alkoxysilane-based modifier to prepare a modified and conjugated diene-based polymer of which at least one terminal is combined with a functional group derived from the modifier; adding an alcohol and an antioxidant to the modified and conjugated diene-based polymer to deactivate the activated organometal part remaining in the modified and conjugated diene-based polymer to prepare a modified and conjugated diene-based polymer which is deactivated; and adding a modification stabilizer of the following Formula 1 to the modified and conjugated diene-based polymer which is deactivated, to perform a condensation reaction of the functional group derived from a modifier in the modified and conjugated diene-based polymer and the modification stabilizer of the following Formula 1: in Formula 1, m is an integer of 0 to 3, R 1 is a functional group having a structure of the following Formula 1-1, and R 2 and R 3 are each independently one of a halogen group, an alkyl group of 1 to 5 carbon atoms, and an alkoxy group of 1 to 5 carbon atoms, where at least one of R 2 and R 3 is the alkoxy group: [Formula 1-1] [R 11 -(R 12) n]- in Formula 1-1, n is 0 or 1, R 11 is an amino group or a four-member to six-member heterocyclic group containing 1 to 3 nitrogen atoms, which is unsubstituted or substituted with at least one substituent selected from the group consisting of an alkyl group of 1 to 10 carbon atoms and a cycloalkyl group of 3 to 10 carbon atoms, and R 12 is an alkylene group of 4 to 10 carbon atoms, which is unsubstituted or substituted with an alkyl group of 1 to 4 carbon atoms. m is an integer of 0 to 3, R 1 is a functional group having a structure of the following Formula 1-1, and R 2 and R 3 are each independently one of a halogen group, an alkyl group of 1 to 5 carbon atoms, and an alkoxy group of 1 to 5 carbon atoms, where at least one of R 2 and R 3 is the alkoxy group: [Formula 1-1] [R 11 -(R 12) n]- in Formula 1-1, n is 0 or 1, R 11 is an amino group or a four-member to six-member heterocyclic group containing 1 to 3 nitrogen atoms, which is unsubstituted or substituted with at least one substituent selected from the group consisting of an alkyl group of 1 to 10 carbon atoms and a cycloalkyl group of 3 to 10 carbon atoms, and R 12 is an alkylene group of 4 to 10 carbon atoms, which is unsubstituted or substituted with an alkyl group of 1 to 4 carbon atoms.