I want to power arduino with powerbank, I connect it by usb, it turns on for a few seconds and turns off. What can I do to prevent the power bank from turning off?

Maybe the Arduino as a load is too low (consuming very low current) so the battery is designed for phone charging and thinks the phone is already full and automatically stops. The problem is inside the battery so you can't do much. Maybe just add more load on side of the Arduino but that just mean the battery will get empty faster so is not a good idea...