This surprises almost every first-time visitor, because no Western metro system does it. It is completely routine, applies to every passenger equally, and is nothing to be anxious about.
What happens#
You queue briefly at the station entrance. Bags go on a belt through an X-ray machine — handbags and daypacks included, not just large luggage. A guard may wave a handheld wand over you.
If you are carrying a drink, expect to be asked to drink some of it. This is a check for flammable liquids, and holding up a sealed factory bottle is usually enough.
Then you collect your bag and continue to the ticket gates. The whole thing takes moments when quiet.
What it means in practice#
Allow extra time at rush hour and at big interchanges. At the busiest stations the queue is the slowest part of the journey. If you are catching a train with a fixed departure, this is the step people forget to budget for.
Do not carry anything obviously prohibited. Flammable liquids, large knives and fireworks are the categories that matter. Ordinary travel possessions are fine — laptops, cameras, tripods, a folding knife in checked-style luggage is where judgement is needed, but nothing a tourist normally carries will cause a problem.
Luggage is allowed. You can take a suitcase on the metro, and plenty of people do. It goes through the scanner like everything else.
Where else you will see it#
The same screening appears at intercity railway stations, airports obviously, and at the entrances to many major tourist sites, government buildings and large shopping centres. Getting used to it early makes the rest of the trip smoother.
Carry your passport. It is required for train travel and hotel check-in, and occasionally asked for at checkpoints.
For station procedure, ticketing and how much time to allow, see the guide to transport in China.