Getting to Xian airport for a 7:35AM flight

What is your flight, international or domestic flight?
The first shuttle bus from the Bell Tower is at around 05:40 and in the early morning, the bus costs about forty minutes.
If you take the domestic flight, you can choose to take the first bus. If you take the international flight, it is suggested to take a taxi and the taxi will cost about 30 minutes to reach the airport.