我解决了这个问题;

// Get cursor current position
                    var cursorPos =
                        _textEditController.selection.base.offset;

                    // Right text of cursor position
                    String suffixText =
                        _textEditController.text.substring(cursorPos);

                    // Add new text on cursor position
                    String specialChars = ' text_1 ';
                    int length = specialChars.length;
                    
                    // Get the left text of cursor
                    String prefixText =
                        _textEditController.text.substring(0, cursorPos);

                    _textEditController.text =
                        prefixText + specialChars + suffixText;
                    
                    // Cursor move to end of added text
                    _textEditController.selection = TextSelection(
                      baseOffset: cursorPos + length,
                      extentOffset: cursorPos + length,
                    );

int position =  controller.selection.base.offset;

简单地得到光标的位置和分裂它的firstpart和secondpart添加文本什么你想要插入
控制器.text=第一部分+“某个文本”+最后部分;