A modified conjugated diene-based polymer comprising: a repeating unit derived from a conjugated diene-based monomer; a functional group derived from an aminoalkoxysilane-based modifier; and a macromonomer, wherein a rate of change of molecular weight distribution calculated by the following Mathematical Formula 1 is 10% or less: Molecular weight distribution increasing rate (%) = [(|PDI 2 -PDI 1 |) /PDI 1]×100 in Mathematical Formula 1, PDI 1 is molecular weight distribution of the polymer before standing, and PDI 2 is molecular weight distribution of the polymer after standing for 90 days at 25°C; wherein the modified conjugated diene-based polymer is obtained by a method including polymerizing a conjugated diene-based monomer, or a conjugated diene-based monomer and an aromatic vinyl monomer in the presence of a polymerization initiator in a hydrocarbon solvent to prepare an active polymer; reacting or coupling the active polymer with an aminoalkoxysilane-based modifier to prepare a modified active polymer; and reacting the modified active polymer and a macromonomer, wherein the macromonomer is a monomer unit of two or more repeating units derived from a polymerization reactive monomer, and may be combined with a polymer chain or a reactive compound through a terminal group, wherein the molecular weight distributions are calculated according to the method as disclosed in the description. wherein a rate of change of molecular weight distribution calculated by the following Mathematical Formula 1 is 10% or less: Molecular weight distribution increasing rate (%) = [(|PDI 2 -PDI 1 |) /PDI 1]×100 in Mathematical Formula 1, PDI 1 is molecular weight distribution of the polymer before standing, and PDI 2 is molecular weight distribution of the polymer after standing for 90 days at 25°C; wherein the modified conjugated diene-based polymer is obtained by a method including polymerizing a conjugated diene-based monomer, or a conjugated diene-based monomer and an aromatic vinyl monomer in the presence of a polymerization initiator in a hydrocarbon solvent to prepare an active polymer; reacting or coupling the active polymer with an aminoalkoxysilane-based modifier to prepare a modified active polymer; and reacting the modified active polymer and a macromonomer, wherein the macromonomer is a monomer unit of two or more repeating units derived from a polymerization reactive monomer, and may be combined with a polymer chain or a reactive compound through a terminal group, wherein the molecular weight distributions are calculated according to the method as disclosed in the description. PDI 1 is molecular weight distribution of the polymer before standing, and PDI 2 is molecular weight distribution of the polymer after standing for 90 days at 25°C; wherein the modified conjugated diene-based polymer is obtained by a method including polymerizing a conjugated diene-based monomer, or a conjugated diene-based monomer and an aromatic vinyl monomer in the presence of a polymerization initiator in a hydrocarbon solvent to prepare an active polymer; reacting or coupling the active polymer with an aminoalkoxysilane-based modifier to prepare a modified active polymer; and reacting the modified active polymer and a macromonomer, wherein the macromonomer is a monomer unit of two or more repeating units derived from a polymerization reactive monomer, and may be combined with a polymer chain or a reactive compound through a terminal group, wherein the molecular weight distributions are calculated according to the method as disclosed in the description.