You should override ActionView::Base.field_error_proc. It's currently defined as this within ActionView::Base:

 @@field_error_proc = Proc.new{ |html_tag, instance| 
   "<p class=\"field_with_errors\">#{html_tag}</p>".html_safe
 }

You can override it by putting this in your application's class inside config/application.rb:

config.action_view.field_error_proc = Proc.new { |html_tag, instance| 
  "#{html_tag}".html_safe 
}

Restart rails server for this change to take effect.

（全文抄袭自 http://stackoverflow.com/questions/5267998/rails-3-field-with-errors-wrapper-changes-the-page-appearance-how-to-avoid-t）

当然，更推荐改成这样

config.action_view.field_error_proc = Proc.new do |html_tag, instance|
  class_attr_index = html_tag.index 'class='

  if class_attr_index
    html_tag.insert class_attr_index+7, 'error '
  else
    html_tag.insert html_tag.index('>'), ' class="error"'
  end
end