Welcome to the Forum @Rex_Wang and thank you for reaching out!
As it stands, there isn’t a built feature to print or export Timelines in Asana. One workaround I would recommend you is to use the browser print feature. Please have a look at this postfor more information.
This a popular topic and we already have two thread requesting these functionalities. I’m listing them below and I would encourage you to upvote them:
I would also recommend you to have a look at the integration Instagantt. You might find it helpful in the meantime!
I hope this helps Rex! Please let me know if you have any follow-up question!