Solution 1:

The answer above do not really solved the problem. They are sending the data as paramters instead of JSON data as the body of the request.

From http://bajiecc.cc/questions/1135255/scrapy-formrequest-sending-json:

my_data = {'field1': 'value1', 'field2': 'value2'}
request = scrapy.Request( url, method='POST', 
                          body=json.dumps(my_data), 
                          headers={'Content-Type':'application/json'} )

Solution 2:

Make sure that each element in your formdata is of type string/unicode

frmdata = {"id": "com.supercell.boombeach", "reviewType": '0', "reviewSortOrder": '0', "pageNum":'0'}
url = "https://play.google.com/store/getreviews"
yield FormRequest(url, callback=self.parse, formdata=frmdata)

I think this will do

In [1]: from scrapy.http import FormRequest

In [2]: frmdata = {"id": "com.supercell.boombeach", "reviewType": '0', "reviewSortOrder": '0', "pageNum":'0'}

In [3]: url = "https://play.google.com/store/getreviews"

In [4]: r = FormRequest(url, formdata=frmdata)

In [5]: fetch(r)
 2015-05-20 14:40:09+0530 [default] DEBUG: Crawled (200) <POST      https://play.google.com/store/getreviews> (referer: None)
[s] Available Scrapy objects:
[s]   crawler    <scrapy.crawler.Crawler object at 0x7f3ea4258890>
[s]   item       {}
[s]   r          <POST https://play.google.com/store/getreviews>
[s]   request    <POST https://play.google.com/store/getreviews>
[s]   response   <200 https://play.google.com/store/getreviews>
[s]   settings   <scrapy.settings.Settings object at 0x7f3eaa205450>
[s]   spider     <Spider 'default' at 0x7f3ea3449cd0>
[s] Useful shortcuts:
[s]   shelp()           Shell help (print this help)
[s]   fetch(req_or_url) Fetch request (or URL) and update local objects
[s]   view(response)    View response in a browser