Dear, change to the express train at the airport to the terminal, then change directly to the subway 13 line (to Huilongguan) inside, then change to Line 4 at Xizhimen Station (to Zhongguancun), and get off at the National Library Station on the way. The express train takes 30 to 40 minutes, and you lie on the subway for about an hour and a half. I hope it helps you. If you don't know, you can ask the uniformed staff in the subway. They are all very enthusiastic.