Thank you Atrol!
I have finally found the resolution:
1.find the file "mantis/config_inc.php",and make modifications as below.
$g_resolution_enum_string = '10:open,20:initial,30:fixed,40:not fixable,50:duplicate';
$g_status_colors = array( 'new' => '#fcbdbd', // red (scarlet red #ef2929)
'assigned' => '#c2dfff', // blue (sky blue #729fcf)
'resolved' => '#d2f5b0', // green (chameleon #8ae234)
'feedback' => '#e3b7eb', // purple (plum #75507b)
'closed' => '#c9ccc4'); // grey (aluminum #babdb6)
$g_bug_readonly_status_threshold = RESOLVED; //You can give a threshold value higher than resolved. and This is the root cause which make the configuration of "workflow" out of work.
2.Of course, you have to modify the lang file if you want localize the status
3.If something happen like this: you update configuration of "workflow" , and then even a developer can't add bugnote , try like this:
log on to MySQL, find the table "mantis_config_table" of your database entity, then delete the row which contains "bug_readonly_status_threshold", or modify the value of the row "bug_readonly_status_threshold" to higher than or equal to resolved.