我的做法不太优雅，但能满足你的需求。

 #filename: forms.py

class ProductForm(Form):
    brands = SelectField(u'brands', 
        choices=[],
        coerce = int)
    ...

    def __init__(self, *args, **kwargs):
        super(ArticleForm, self).__init__(*args, **kwargs)
        self.brands.choices = [(b.id, b.name) for b in Brand.query.all()]

class ModelForm(Form):
    models = SelectMultipleField('models', 
        choices = [],
        option_widget = widgets.CheckboxInput(),
        widget = widgets.ListWidget(prefix_label = False))

views.py

 # filename: views.py

@app.route('/edit')
def product_edit():
    form = ProductForm()
    if form.validate_on_submit():
        """ request.form是一个MultiDict,request.form['models']仅仅能得到models列表的第一个元素，此处使用request.form.getlist()来获取整个列表。
        """
        return render_template_string('{{data}}', request.form.getlist('models'))
    return render_template('product_edit.html',form = form)

# function for Ajax
@app.route('/getmodel/<int:id>')
def model(id):
    brand = Brand.query.get(id)
    form = ModelForm()
    if brand is not None and brand.models:
        form.models.choices = [(c.id, c.name) for c in brand.models]
        return render_template_string('{{ form.models(multiple=True) }}', form = form)
    return u'暂无型号'

the template product_edit.html

 <div>{{ form.brands }}</div>
<div id="m"></div>
<script>
    $(function(){
        $("#brands").change(function(){
            id = $(this).val();
            url = "{{ url_for('model', id=0) }}".replace("/0","/"+id)
            $.get(url, function(result){
                $("#m").html(result);
            })
        })
    })
</script>

不用Ajax就POST到Server端，重新赋值form.Filed.Choices,再转跳回来呗~