When an administrator (except user 1) editing a user how to hide "administrator" under role in a user edit page in D7 site? or how to prevent assign a role of administrator while editing a user?
I believe the RoleAssign module can help you with that:
RoleAssign specifically allows site administrators to further delegate the task of managing user's roles while withholding the Administer permissions permission.
RoleAssign introduces a new permission called Assign roles. Users with this permission are able to assign selected roles to still other users. Only users with the Administer permissions permission may select which roles are available for assignment through this module.
Use the role delegation module.
It gives new permissions to restrict the roles that are allowed to be assigned.