Software　as　a　Service　(SaaS)　provides　the　same　instance　of　an　application　to　multiple　customers　typically　in　a　single-instance　multi-tenant　architecture　mode,　achieving　a　low　cost,　economies　of　scale.　However,　SaaS　applications　also　face　the　three　most　important　challenges　:　multi-tenant,　configurable　and　extensible.　While　the　current　SaaS　applications　delivers　standardized　functions,　this　architecture　does　not　match　the　dynamic　business　environment　which　demands　flexibility　and　innovative　approach　to　interactive　requirements.　This　paper　presents　an　open　architecture　to　build　SaaS　solutions,　the　combination　of　SOA　and　EDI　reference　models　to　create　a　clear　reference　architecture,　automatic　path　between　the　business　specification　layer　and　the　technical　implementation　layer,　tenants　can　take　part　in　the　design　of　business　processes　and　we　describe　how　individual　services　join　the　SaaS　applications.　©　2010　IEEE.