Theoretical investigation of low-lying electronic states and B3Σ−u−X3Σ−g transition properties of selenium dimer using size-extensivity singly and doubly excitation multireference configuration interaction theory with nonrelativistic all-electron basis set and relativistic effective core potential plus its split valence basis set is presented in this paper. The spectroscopic constants of ten low-lying Λ−S bound states have been obtained and compared with experiments. Spin-orbit calculations for coupling between B3Σ−u sates and repulsive 1Πu, 5Πu states have been made to interpret the predissociation mechanisms of the B3Σ−u state. The lifetimes of B3Σ−u (ν = 0 ∼ 6) have been calculated with scalar relativistic effects included or excluded, respectively, and reasonably agree with experimental values.