Compared with isoalloxazine, the core chromophore of biologically important flavins, alloxazine exhibits much lower fluorescence quantum yield and larger intersystem-crossing quantum yield. However, its efficient radiationless relaxation pathways are still elusive. In this work, we have used the QM(MS-CASPT2//CASSCF)/MM method to explore the mechanistic photophysics of alloxazine chromophore in aqueous solution. On the basis of the optimized minima, conical intersections, and crossing points in the lowest (1)pi pi*, (1)n pi*, (3)pi pi*, and (3)n pi* states, we have proposed three energetically possible nonadiabatic relaxation pathways populating the lowest (3)pi pi* triplet state from the initially populated excited (1)pi pi* singlet state. The first is the direct (1)pi pi* -> (3)pi pi* intersystem crossing via the (1)pi pi*/(3)pi pi* crossing point. The second is an indirect (1)pi pi* -> (3)pi pi* intersystem crossing relayed by the dark (1)n pi* singlet state. In this route, the (1)pi pi* system first decays to the (1)n pi* state via the (1)pi pi*/(1)n pi* conical intersection, followed by an (1)n pi* -> (3)pi pi* intersystem crossing at the (1)n pi*/(3)pi pi* crossing point to arrive at the final (3)pi pi* state. The third is similar to the second one; but its intersystem crossing is relayed by the (3)n pi* triplet state. The (1)pi pi* system first decays to the (3)n pi* state via the (1)pi pi*/(3)n pi* crossing point; the generated (3)n pi* state is then de-excited to the (3)pi pi* state through the (3)n pi* -> (3)pi pi* internal conversion at the (3)n pi*/(3)pi pi* conical intersection. According to the classical El-Sayed rule, we suggest the second and third paths play a much more important role than the first one in the formation of the lowest (3)pi pi* state.