The photodissociation of CH4/Pt(lll) is studied by density functional theory and the state-averaged complete active space self-consistent field (SA-CASSCF) method using a cluster model Pt-n (n = 1,4,6,7,10). With the small clusters (n less than or equal to 4), the equilibrium molecule-surface distances (H3CH-Pt) are less than 2.3 Angstrom and the binding energies are 4-14 kcal/mol, the order of the chemisorption. With larger clusters, the molecule-surface distance and the binding energy are calculated to be 3.00 Angstrom and 0.67 kcal/mol, respectively, of the order of the physisorption, which coincides with the experiments. The SA-CASSCF calculations verify that, in spite of the weak interaction between CH4 and Pt-n in the ground state, the first excited state of CH4 (Rydberg type) interacts with Pt-n unoccupied states strongly, resulting in the charge-transfer state and finally leading to the dissociation to CH3+H(-Pt); on the Pt(lll) surface, the excitation energy to the Rydberg state of CH4 decreases by similar to 3 eV compared to that in an isolated CH4 molecule. These results support the experimental results that the direct excitation of CH4 is invoked on the Pt(lll) surface by irradiation of the 193 nm photon, leading to the dissociation to CH3 and H.