The photochemistry of carbonyl compounds containing β-phenyl rings has attracted increasing attention, for it is regarded as a representative template for the triplet n,π* quenching of carbonyl groups. In gas phase and in methanol, the ground states and excitation states of β-benzylacetophenone were investigated and its absorption and emission spectra were simulated. Besides, the luminescence mechanisms were also elucidated from molecular orbitals. The theoretical results revealed that: (1) Its ground-state structure in methanol is very close to the geometry in gas phase except for the bond distances around the carbonyl group; (2) In methanol, the S1 state of β-benzylacetophenone can not maintain a planar configuration, and the α C—C bond is extended significantly; (3) The absorption spectrum of β-benzylacetophenone is very weak in gas phase, but it is very strong in methanol; (4) The fluorescence spectrum of β-benzylacetophenone in gas phase is also different from that in methanol; (5) In gas phase, the strongest emission peak of the fluorescence spectrum is blue-shifted to about 228.67 nm, and its oscillator strength (f=0.306 1) increases significantly; (6) The fluorescence spectrum is an inverse process of the absorption spectrum from the viewpoint of molecular orbitals; (7) In phosphorescence spectrum, there are two relatively strong emission peaks at 252.58 and 246.04 nm in gas phase, but there is only one very strong emission peak at 258.88 nm in methanol.