There are lots of guides about this online, but they all seem rather outdated. Does anyone know if it's possible for a tourist to buy a SIM card at Beijing airport (terminal 2)? What should I keep in mind? My phone is unlocked. I'm staying for 3 weeks.
At the airport
From recent sources, there should be areas where you can buy a SIM card in terminal 3, none in terminal 2.
A TripAdvisor post from 2 years ago:
I believe that the only place to buy a SIM card at the Beijing airport is at terminal 3. It is in the area where you wait for your luggage. It is a China Mobile counter. If that does not work for you then you need to buy somewhere in town.
An alternative is to get a SIM card at another place with China unicom.
This can be delivered to your hotel from free.
From China Unicoms' website:
Do you sell the prepaid SIM card at the airport?
Currently, we don’t have resellers at airport; however, we have more than 100 hotels in Beijing, Shanghai and Chengdu that sell our SIM cards. The best way to buy the SIM card is to order it on www.mychinaunicom.com, and have it delivered to your hotel for free!
Pick up at airport
An alternative option is to pick up this SIM card at the airport (ordered in advance), they offer pick ups at Beijing Capital International Airport (PEK) T2 (terminal 2).
This service is in date as the most recent review is 20th September 2019.
I believe this is your best option.
I arrived at PEK Terminal 2 on Air France about a month ago, and I saw two stalls in the baggage claim area selling SIM cards. I don't remember the exact pricing as I decided to go to a China Unicom store in the city instead, but it was definitely lower than the one offered by klook from Daniil's link above (think about 200CNY for unlimited data and 100 domestic minutes). These stalls didn't look very "official", though, but since the staff speak English, foreign travellers might still prefer getting a SIM card here over a store in the city.
I seem to remember a "proper" China Mobile store just outside of the baggage claim area, i. e. after customs (confirmed by Baidu maps, apparently open 09:00-20:00), which might have lower pricing as well.
By the way, mychinaunicom.com seems to be a website targeted at foreign visitors and has much higher prices than the actual China Unicom. For example, I paid 160CNY for 20GB and 300 minutes, a person on TripAdvisor very recently got the same for just 99CNY, while mychinaunicom charges ~650CNY for 6GB and 250 minutes.