AWS S3 JavaScript SDK getSignedUrl returns url UnauthorizedAccess

User sdk version : aws-sdk-2.283.1.min.js

I use like this:

var myCredentials = new AWS.Credentials("key",
    "secret");
var config = new AWS.Config();
config.update({
    region: "cn-north-1",
    credentials: myCredentials
});var s3 = new AWS.S3(config);
var p2 = {
    Bucket: 'equipment-firmware', Key: 'test/func.jpg', ResponseContentDisposition: 'attachment', Expires: 6000
};

s3.getSignedUrl('getObject', p2, function (err, url) {
    if (err) {
        console.log(err, err.stack);
    } else {
        console.log('The URL is', url);
        window.open(url);
    }
});

But use return url, in browser status is 401 and show this:

<Error>
<Code>UnauthorizedAccess</Code>
<Message>You are not authorized to perform this operation</Message>
<RequestId>649CEE7A351D2549</RequestId><HostId>RUWINnJ4IHsN7Jicb6oI7L87ZfYFC5J+c3fmLgwkTqJ+G+AvvAD7scLZDKUBFzZxC+Bg7HaNua4=</HostId>
</Error>