Density　Functional　Theory　is　used　to　calculate　the　epoxidation　of　propylene　on　the　dimer　MoOx　by　Gaussian　09　software.　We　calculate　the　dehydrogenation　process　of　propylene　and　the　formation　process　of　propylene　oxide.　It　is　found　that　the　activity　of　O　in　Mo-O-Si,　which　is　beneficial　to　the　dehydrogenation　process　of　propylene,　is　high.　Furthermore,　we　establish　a　dimer　MoOx　model　with　defect　sites,　and　find　that　the　defect　sites　in　the　dimer　MoOx　can　activate　O-2　(O-2(2-))　effectively.　Consequently,　the　catalyst　containing　defect　sites　is　conducive　to　the　selectivity　of　propylene　oxide.