The　trust　degree　evaluation　problem　is　researched　in　this　paper.　Based　on　the　uncertainty　and　fuzziness　of　trust,　a　method　for　discrete　presentation　of　trust　is　proposed.　With　the　powerful　inference　ability　of　Bayesian　networks,　the　Bayesian　network　model　for　composite　service　is　presented　and　the　rules　for　translation　of　the　composite　service　model　to　Bayesian　networks　are　designed.　The　trust　degree　evaluation　method　for　composite　services　based　on　the　forward　reasoning　mechanism　of　Bayesian　networks　is　designed.　The　using　method　of　the　Bayesian　network　model　and　the　evaluation　method　is　illustrated　by　a　composite　service　example.　Because　the　trust　degree　evaluation　method　is　based　on　uncertainty　and　fuzziness　of　trust　concept,　the　method　is　true　of　essence　of　trust　and　easy　to　implement.