//按楼主的写法改的

procedure TForm1.TreeView1DragOver(Sender, Source: TObject; X, Y: Integer;
  State: TDragState; var Accept: Boolean);
begin
  Accept := Source is TTreeView;
end;

procedure TForm1.TreeView1DragDrop(Sender, Source: TObject; X, Y: Integer);
var
  I: integer;
  sNode, tNode : TTreeNode;
begin
  tNode := TTreeView(Source).GetNodeAt(X, Y);
  for I := 0 to TTreeView(Source).SelectionCount-1 do begin
    Showmessage(inttostr(i));
    sNode := TTreeView(Source).Selections[I];
    sNode.MoveTo(tNode, naAddChild);
  end;
end;