The aging of modified asphalt includes modifier and asphalt aging. Combining with the changes of macroscopic properties index for modified asphalt before and after aging, aging mechanism of modified asphalt was studied by gel permeation chromatography(GPC) and grey relevant methods. And the results indicated that changes in content, molecular weight and dispersion degree of asphalt before and after aging are the most significant factors influencing macroscopic properties of modified asphalt. Asphalt content is the most significant factor influencing penetration, toughness and tenacity, ductility(5 °C) and quality; Weight average molecular weight of asphalt is the most significant factor influencing viscosity(135°C); Dispersion degree of asphalt is the most significant factor influencing elastic recovery. Conclusion provides the test basis for the evaluation of anti-aging properties of modified asphalt.