Blank site when importing

When I am importing a XML file in import. I get this error Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 55018 bytes) in /www/wordpress/wp-content/plugins/wordpress-importer/parsers.php on line 460

My file is 70 MB, I have change the php.ini to 2048MB. What is wrong

http://ww.wp.xz.cn/extend/plugins/wordpress-importer/