找了半天找到解决办法了，在非只读模式下打开邮箱，然后按照API 给出的在 fetch 方法的第二个参数中添加 markseen:true 就行了，示例如下：

var Imap = require('imap'),
    inspect = require('util').inspect;

var imap = new Imap({
    user: '[email protected]',
    password: 'XXXXX',
    host: 'imap.gmail.com',
    port: 993,
    tls: true
});

function openInbox(cb) {
    imap.openBox('INBOX', false, cb);
}

imap.once('ready', function() {

    openInbox(function(err, box) {
        if (err) throw err;
        imap.search([ 'UNSEEN', ['SINCE', 'May 20, 2014'] ], function(err, results) {
            console.log('unseen mail count: ' + results.length);
            if (err) throw err;
            var f = imap.fetch(results, { bodies: '', markSeen : true });

            f.on('message', function(msg, seqno) {
                console.log('Message #%d', seqno);
                var prefix = '(#' + seqno + ') ';
                msg.on('body', function(stream, info) {

                    if (info.which === 'TEXT')
                        console.log(prefix + 'Body [%s] found, %d total bytes', inspect(info.which), info.size);
                    var buffer = '', count = 0;
                    stream.on('data', function(chunk) {
                        count += chunk.length;
                        buffer += chunk.toString('utf8');
                        if (info.which === 'TEXT')
                            console.log(prefix + 'Body [%s] (%d/%d)', inspect(info.which), count, info.size);
                    });
                    stream.once('end', function() {
                        console.log(buffer);
                        if (info.which !== 'TEXT')
                            console.log(prefix + 'Parsed header: %s', inspect(Imap.parseHeader(buffer)));
                        else
                            console.log(prefix + 'Body [%s] Finished', inspect(info.which));
                    });


                });
                msg.once('attributes', function(attrs) {
                    console.log(prefix + 'Attributes: %s', inspect(attrs, false, 8));
                });
                msg.once('end', function() {
                    console.log(prefix + 'Finished');
                });
            });



            f.once('error', function(err) {
                console.log('Fetch error: ' + err);
            });
            f.once('end', function() {
                console.log('Done fetching all messages!');
                imap.end();
            });
        });
    });
});

imap.once('error', function(err) {
    console.log(err);
});

imap.once('end', function() {
    console.log('Connection ended');
});

imap.connect();