According　to　the　study　of　biological　vision,　human　visual　attention　is　aroused　by　observer＇s　tasks　and　it　is　a　top-down　control　mechanism.　A　top-down　visual　attention　model　based　on　similarity　distance　is　proposed　in　this　study.　Top-down　visual　attention　model　is　realized　by　using　similarity　distance　to　obtain　visual　expectation　based　on　improved　Itti＇s　model.　A　task　map　is　selected　by　using　improved　Itti＇s　model　and　user＇s　observation　tasks,　and　then　to　compute　the　similarity　distances　between　task　map　and　saliency　regions　from　improved　Itti＇s　model.　The　weights　of　intensity,　color　and　orientation　are　adjusted　with　similar　distances　in　order　to　measure　the　saliency　in　top-down　visual　attention　model.　After　that,　area　enhancement　factor　is　utilized　to　modify　the　saliency　and　compute　the　visual　expectation.　Region　of　interest　(ROI)　is　extracted　by　combining　visual　expectation　with　saliency　region.　The　experimental　results　show　that　the　proposed　model　is　available　and　the　ROI　results　comply　with　biological　attention　mechanism.