(1) performing a Fourier transform infrared spectroscopy (FTIR) test on unaged SBS-modified asphalt samples, detecting the absorbance of infrared spectrum characteristic functional groups polystyrene and polybutadiene of unaged SBS-modified asphalt at 699 cm⁻¹ and 966 cm⁻¹, calculating a value of A₆₉₉/A₉₆₆, and recording the value as an unaged SBS copolymer index I_B0/S0; and detecting the absorbance of the unaged SBS-modified asphalt at 1030 cm⁻¹, 1456 cm⁻¹, 1600 cm⁻¹, and 1700 cm⁻¹, calculating functional group indexes of sulfoxide group, aliphatic group, aromatic group and carbonyl of the unaged SBS-modified asphalt, and recording the same as indexes of unaged neat asphalt SI₀=A₁₀₃₀/ΣA_650˜2000, I_B,aI₀=A₁₄₅₆/ΣA_650˜2000, ARI₀=A₁₆₀₀/ΣA_650˜2000, and CI₀=A₁₇₀₀/ΣA_650˜2000;

A₆₉₉, A₉₆₆, A₁₀₃₀, A₁₄₅₆, A₁₆₀₀, and A₁₇₀₀ referring to corresponding peak areas at infrared spectrum wave numbers of 699 cm⁻¹, 966 cm⁻¹, 1030 cm⁻¹, 1456 cm⁻¹, 1600 cm⁻¹, and 1700 cm⁻¹, and ΣA_650˜2000 referring to a sum of peak areas at infrared spectrum wave numbers of 650 cm⁻¹−2000 cm⁻¹;

(2) performing the FTIR test on laboratory-fabricated SBS-modified asphalt samples or in-service SBS-modified asphalt pavement samples; detecting the absorbance of the laboratory-fabricated SBS-modified asphalt or in-service SBS-modified asphalt pavement samples at 699 cm⁻¹ and 966 cm⁻¹, calculating a value of A′₆₉₉/A′₉₆₆, and recording the value as an aging SBS copolymer index I_B/S; and detecting the absorbance of the laboratory-fabricated SBS-modified asphalt or in-service SBS-modified asphalt pavement samples at 1030 cm⁻¹, 1456 cm⁻¹, 1600 cm⁻¹, and 1700 cm⁻¹, calculating functional group indexes of sulfoxide group, aliphatic group, aromatic group and carbonyl of the aged SBS-modified asphalt, and recording the same as indexes of aged neat asphalt SI=A′₁₀₃₀/ΣA′_650˜2000, I_B,aI=A′₁₄₅₆/ΣA′_650˜2000, ARI=A′₁₆₀₀/ΣA′_650˜2000, and CI=A′₁₇₀₀/ΣA′_650˜2000; and

(3) delineating three aging stages of SBS-modified asphalt according to changes of infrared spectrum characteristic functional group indexes obtained by actual detection and calculation:

a, an SBS polymer swelling stage:

 &

b, an SBS polymer degradation stage:

 &

 and

c, an asphalt component imbalance stage:

 &