I found the answer, it was just a problem with the jquery part. The parameter contentType of $.ajax defaults to "application/x-www-form-urlencoded". You need to set this parameter to application/json.

$.ajax({
        url: "{{ url_for('main.getjson') }}",
        type: "POST",
        data: JSON.stringify({
            "n1": "test1",
            "n2": "test2",
            "n3": "test3"
        }),
        contentType: "application/json",
        dataType: "json",
        success: function(data){
            var a = data.user
            var texthtml = "<p>" + a + "</p>"
            $("#result").html(texthtml)
        }
    });

Reference: https://flask.readthedocs.io/...
http://stackoverflow.com/ques...

jquery’s ajax does not require JSON.stringify when sending data. It will be processed automatically.

According to your description, you have found the problem, why not continue to try it, or read the documentation

@app.route('/api', methods=['POST'])
def api():
    a = request.json
    b = request.get_data()
    c = request.values
    print a
    print b
    print c
    if a:
        return 'ok'
    return 'error one'

Output results

None
{"id":1}
CombinedMultiDict([ImmutableMultiDict([]), ImmutableMultiDict([('{"id":1}', u'')])])

request.json is very strange. It really has no data, but it can be used. The reason why it is not useful here is as follows:

json If the mimetype is application/json this will contain the parsed
JSON data. Otherwise this will be None. The get_json() method should
be used instead.
But when I use get_json(), it ruthlessly reports an AttributeError : 'Request' object has no attribute 'get_json'

So I only successfully used request.json once, and then never succeeded again, because it is magical, if I can find an alternative to request.json.