The modified Flory-Huggins formula was adopted to calculate the free energy in mixing old asphalt with rejuvenator, and discussed their phase behavior. The solubility parameters and ratio of aromatic carbon of three aged asphalts are more than that of their materials. The mixing free energy of Caltex aged asphalt and rejuvenator R1 is above 0.0 (ΔGM＞0) at 25 ℃, their process of mixing is not spontaneity, so their mixing can’t form the homogeneous phase system. The mixing free energies of R2, R3, composite rejuvenator and aged asphalt are all below 0.0 (ΔGM＜0) at 25 ℃, their processes of mixing are spontaneity, so their mixing can form the homogeneous phase system.