create table tb_test( id bigint, filed_test enum('aa','bb') DEFAULT NULL);
alter table tb_test modify filed_test enum('aa','bb','cc') DEFAULT NULL;

------解决方案--------------------

SQL code
mysql> create table tb_test(
    -> id bigint,
    -> filed_test enum('aa','bb') DEFAULT NULL
    -> );
Query OK, 0 rows affected (0.13 sec)

mysql> alter table tb_test modify filed_test enum('aa','bb','cc') DEFAULT NULL;
Query OK, 0 rows affected (0.11 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show create table tb_test;
+---------+---------------------------------
-----------------------------------------+
| Table   | Create Table
                                                                             |
+---------+---------------------------------
-----------------------------------------+
| tb_test | CREATE TABLE `tb_test` (
  `id` bigint(20) DEFAULT NULL,
  `filed_test` enum('aa','bb','cc') DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+---------+---------------------------------
-----------------------------------------+
1 row in set (0.00 sec)

mysql>