Abstract: The density functional theory (DFT) is used to describe the perpendicular phase separation of binary mixed polymer brush grafted on a surface immersing in different solvents. The DFT predicts that: for binary compatible polymer brushes with the same chain length and interaction potential, the grafted layer shrinks in non-selective poor solvent and swells in non-selective good solvent with no perpendicular phase separation. When the brushes are exposed to a selective solvent, perpendicular phase separation induced by the solvent occurs, the solvophilic chains swell to the upper layer while the solvophobic chains twist at the bottom, and the surface modified by grafted polymer brush is solvophilic. The degree of phase separation is decided by the selectivity of solvent and the incompatibility of two polymers.