Topological insulators are new class of quantum materials,which have insulating energy gaps in the bulk,but exhibit gapless edge states or surface states that are protected by time-reversal symmetry at boundary.It was theoretically predicted and experimentally confirmed that the binary tetradymites Bi2Te3,Bi2Se3,and Sb2Te3 are three-dimensional topological insulators.In this work,we demonstrate by first-principle approach that the ignored Sb2Se3,although with relatively smaller spin-orbital coupling strength,can also exhibit topologically protected surface states with a bulk gap of 0.19 eV,as long as the van der Waals interaction is explicitly included in the calculations.Detailed analysis of the band structures of Sb2Se3 thin films indicates that the non-trivial surface state appears at a critical thickness of six quintuple layers.