The bandedge electronic structure including the optical bandgap, band-tail states, and deep/shallow donor and acceptor levels in Cu₂ZnSnS₄ semiconductor was analyzed by absorption, photocurrent and photoluminescence spectroscopy, and the theoretical reports. It is revealed that the SnZn-related defect in Cu₂ZnSnS₄ with abundant defect states is one of the key factors affecting the band-edge electronic structure. High concentration of the neutral defect cluster [2CuZn+SnZn] can narrow the band gap substantially, while the partially-passivated （ionic） defect cluster [CuZn+SnZn] is the main deep donor defect. A large number of band-tail states are responsible for the obvious red-shift of the bandedge-related photoluminescence transition energy. These detrimental defects related to SnZn can be effectively suppressed by properly reducing the Sn content in the copper-poor and zinc-rich growth condition, which also avoids the narrowing of the optical bandgap of the Cu₂ZnSnS₄ absorption layer.