In concert with the latest laser-induced fluorescence (LIF) experiment [Wei et al. J. Phys. Client. A 2008, 112, 4727], we investigated the photodissociation mechanics of the benzoic acid monomer (BAM) with alpha C-O fission by means of state-of-the-art ab initio calculations. Complete active space self-consistent-field (CASSCF) and multi reference CASSCF second-order perturbation theory (MSCASPT2) calculations were performed on the ground and a number of low-lying excited states of BAM. Our calculations indicated that alpha C-O fission from the S, state is in competition with the fission from the T-2 state upon the 266-284 nm wavelength photon. This differs from the conclusion of the previous theoretical investigation and clarified the vague experimental conclusion made earlier. According to Our calculations, alpha C-O fission mainly occurs at the T, state upon photoexcitation at 284-294 nm, and the photon with a wavelength longer than 294 nm is unable to present the alpha C-O fission. This conclusion agrees with the LIF experimental observation.