varchar2(4000) abc;
intert into table_name(abc) values('这里有1500个汉字……');

SQL> select * from v$nls_parameters where parameter='NLS_CHARACTERSET';

PARAMETER
------------------------------------------------
VALUE
------------------------------------------------
NLS_CHARACTERSET
AL32UTF8

SQL> SHUTDOWN IMMEDIATE;  
SQL> STARTUP MOUNT; 
SQL> ALTER SYSTEM ENABLE RESTRICTED SESSION; 
SQL> ALTER SYSTEM SET JOB_QUEUE_PROCESSES=0; 
SQL> ALTER DATABASE OPEN; 
SQL> ALTER DATABASE CHARACTER SET AL32UTF8/ZHS16GBK;
SQL> SHUTDOWN IMMEDIATE;
SQL> STARTUP;