You should start from Beijing Capital International Airport to Xian International Airport. The ticket with 70% discount costs CNY320. It takes about one hour and 50 minutes. After you arrive at Xian airport, you can take the airport bus to go to the downtown of Xian City. At Xian Railway Station,you may take the Special Bus No.306 (now Tourism Bus No.5, i.e. 游5 in Chinese）to go there.