这是我的解决方案。
在我的 form.html.twig 文件顶部:
{% form_theme form with 'MyBundle:Activity:Form/fields.html.twig' %}
现在进入fields.html.twig,我自定义了form_label:
{% extends 'form_div_layout.html.twig' %}
{% block form_label %}
{% spaceless %}
{% if not compound %}
{% set label_attr = label_attr|merge({'for': id}) %}
{% endif %}
{% if required %}
{% set label_attr = label_attr|merge({'class': (label_attr.class|default('') ~ ' required')|trim}) %}
{% endif %}
{% if label is empty %}
{% set label = name|humanize %}
{% endif %}
<label{% for attrname, attrvalue in label_attr %} {{ attrname }}="{{ attrvalue }}"{% endfor %}>{{ label|trans({}, translation_domain) }}{% if attr.note is defined %} <span style="font: 11px normal; font-family: arial;">({{ attr.note }})</span>{% endif %}</label>
{% endspaceless %}
{% endblock form_label %}