TreeXML.TVExpand(TVI_ROOT, TVE_EXPAND);

treeview.FullExpand;

procedure FullExpand(N: THandle);
var a: Cardinal;
   b: Boolean;
begin
 b := N = 0;
 if  b then
     N := TreeXML.TVRoot;
 while N <> 0 do
 begin
     a := TreeXML.TVItemChild[N];
     if  a > 0 then FullExpand(a);
     TreeXML.TVExpand(N, TVM_EXPAND);
     N := TreeXML.TVItemNext[N];
 end;
end;
procedure TForm1.OpenTestClick(Sender: PObj);
begin
FullExpand(TreeXML.TVRoot);
end;