scrapy怎么多次向一个url请求信息，然后获取返回数据? 400 报错

scrapy怎么多次向一个url请求信息，然后获取返回数据? 400 报错 大家好，我刚开始学习使用scrapy，希望能请教一个问题。

这是我的spiders文件夹中爬虫文件的代码

class PabhSpider(CrawlSpider): name = 'pabh' allowed_domains = ['xxx']

def start_requests(self):
    url = 'http://xxx'
    num1 = '01'
    formdata = {
        "depart":num,
        "years":'2014'
    }
    return [FormRequest(url=url,formdata=formdata,method='get',callback=self.parse)]


def parse(self, response):
    item = XXXItem()
    hxs = Selector(response)
    item['bh'] = hxs.xpath('/html/body/form/p/font/select[3]/option/@value').extract()
    yield item

    num = ['02','03','04','05','06','07','08','09','10','11','12','13','14','21','31','40','51','61']

    for x in  num:
        url = 'http://xxx'
        formdata={
            "depart":x,
            "years":'2014'
        }
        yield FormRequest(url=url,formdata=formdata,method='get',callback=self.parse)</pre>