Take Metro Line 2 to the west at Beijing Railway Station and take it at Xuanwumen Station, then transfer to Metro Line 4 at Xuanwumen Station (pay attention to the sign of transfer), take Metro Line 4 to the north and take it at the National Library Station, and the National Library will exit at the west. The total time is about 40-50 minutes. Just buy a ticket (2 yuan), and you don't have to pay for a transfer.