In this study, the photoinduced ultrafast deactivations of O-6-methylguanosine in aqueous solution are mapped by employing multireference methods with implicit solvation model on a reduced model compound 9-methyl-O-6-methylguanine (9Me-6MeGua). Although four S-1/S-0 minimal energy conical intersections (MECIs) are located, energy profiles indicated that only two of them are involved in excited-state decay processes and account for the C2 and C6 reaction coordinates, respectively. To reach both the involved MECIs, 9Me-6MeGua needs to surmount an energy barrier. The computed energy barrier of the C6 reaction coordinate is extremely low and should be responsible for the experimentally observed fast internal conversion (IC) process (similar to 0.95 ps), whereas the C2 reaction coordinate accounts for the slow IC process (similar to 41 ps). Eventually, we deliver a detailed mechanism for ultrafast deactivations of 6MeGuo, which explains well the recent experimental results [Ashwood, B.; et al. J. Phys. Chem. Lett. 2017, 8, 4380-4385] and revises the proposed mechanism.