您可以使用EM_CHARFROMPOS来确定鼠标光标指向的字符位置：

var
  Pt: TPoint;
  Pos: Integer;
begin
 Pt := Memo1.ScreenToClient(Mouse.CursorPos);
 if (Pt.X >= 0) and (Pt.Y >= 0) then begin
   Pos := LoWord(Memo1.Perform(EM_CHARFROMPOS, 0, MakeLong(Pt.x, Pt.Y)));
   Memo1.SelLength := 0;
   Memo1.SelStart := Pos;
   Memo1.SelText := '.. insert here ..';
 end;
end;

如果你想把你的字符串放在备忘录的插入符号位置，你可以使用下面的代码：

procedure TForm1.InsertStringAtcaret(MyMemo: TMemo; const MyString: string);
begin
  MyMemo.Text :=
  // copy the text before the caret
    Copy(MyMemo.Text, 1, MyMemo.SelStart) +
  // then add your string
    MyString +
  // and now ad the text from the memo that cames after the caret
  // Note: if you did have selected text, this text will be replaced, in other words, we copy here the text from the memo that cames after the selection
    Copy(MyMemo.Text, MyMemo.SelStart + MyMemo.SelLength + 1, length(MyMemo.Text));

  // clear text selection
  MyMemo.SelLength := 0;
  // set the caret after the inserted string
  MyMemo.SelStart := MyMemo.SelStart + length(MyString) + 1;
end;

你可以在“.Lines”成员的底部添加一个字符串：MyMemo.Lines.add('MyString') .
你也可以 * 替换 * 你想要的“Lines”中任何（已经存在的）位置的字符串：MyMemo.Lines[2] := 'MyString'。
最后，你可以 * 插入 * 任何你想要的地方：MyMemo.Lines.Insert(2, 'MyString')

这很简单（NGLN的答案）：

MyMemo.SelText := MyString;