XQA　(an　XML　Query　Algebra),　as　well　as　a　intermediate　functional　language　named　FXQL　(a　Functional　XML　Query　Language)　for　implementation　of　XQuery　language,　is　introduced　to　support　optimizing　for　complex　XML　query.　Furthermore,　a　decorrelation　method　base　on　XQA　is　put　forward,　which　helps　to　use　efficient　Join　algorithms　to　speed　up　query　execution　by　rewriting　the　nested　XML　query　with　Join　operator.　Based　on　an　extended　XDM　model　with　list　structure,　the　query　rewriting　is　implemented　by　a　program　transformation　on　FXQL　program　and　easy　to　be　combined　with　the　various　optimizations　for　functional　language.　Results　of　experiments　demonstrate　the　effect　of　this　optimization.