A density functional approach to the description of the selective adsorption of a binary mixture of small molecules on a surface grafted with the square-well chains is proposed. The excess Helmholtz free energy functional of the system is divided into two contributions from the hard-sphere repulsion and the square-well attraction respectively. In the bulk phase, the equation of state of Liu and Hu (Liu H L, Hu Y 1996 Feuid Phase Equilibra 122 75) is used to calculate the free energy of the repulsive part and the equation of state of Li and He (Li J L and He H H 2009 Feuid Phase Equilibria 276 57) is used to calculate the free energy of the attractive part. A simple weighted density approximation is adopted on both parts to construct the functional. Using this theoretical approach, the structures of the grafted polymers and the selective adsorptions of binary square-well fluids on the grafted layer at different temperatures are investigated. The theory predicts that the brush thickness increases linearly with the grafting density but non-linearly with temperature and the brush thickness tends to sateration at high temperature. Given the chain length and grafting density, the theoretical calculation also reveals that the polymer brush has strong selective adsorption cap ability at low temperature and the cap ability will weaken greatly when the polymer brush is heated above the temperature of sateration.