The diffuse bands B３Σ-u→X３Σ-g(ν′≥18,0) of diatomic molecule S2 observed in the experiment are investigated. The electronic potential curves,including the spin-orbit coupling (SOC) effect of B３Σ-u and repulsive 1５Πu， 23Σ+u states are calculated. For the diffuse bands beginning at (18,0), a point of view different from others' results is presented in this work. Our results indicate that the SOC induced predissociation between B３Σ-u and 1５Πu， 23Σ+u plays the key role in the diffusion of spectra. Comparison with experimental results shows good agreement.