It takes around 40 mins from the Capital Airport to the Summer Palace, if traffic is ok. And you may need at least 30 mins to go through the immigration and claim luggage. So, it is unlikely to get to the Summer Palace 1h10m after your landing, unless you are really lucky.

Let’s calculate that. It takes about 1-1.5 hours to handle the check-out procedure of the international flight, and then another 50 minutes' drive is needed for the transfer from the airport to the Summer Palace which is closed at 18:00. In case there is the traffic jam, more time will be spent. So I don’t think you can make it.