In order to reveal the epithermal aging mechanism of Gussasphalt from microcosmic perspective, real time scanning on change rule of molecular groups and light components of Gussasphalt modified asphalt was conducted by Fourier transform infrared spectroscopy(FTIR) and thermal gravimetric analysis(TG) at different epithermal temperature. The impact of volatilization and oxidation on the aging of the modified asphalt at epithermal temperature was investigated. The results show that volatilization at epithermal temperature has a great impact on epithermal aging，which also runs through the whole aging process. At the same time, the acting time of oxidation in asphalt aging process is determined by oxygen concentration. Oxidation acts in preliminary process and volatilization of light components acts mainly in late process under high oxygen concentration.