Online buy

The same counter for passengers, the queue is also the same. But if you have already bought online, your seat or bed will be guaranteed. Otherwise there might be no tickets left for you. For Chinese, they could buy at the ticket machines, they do not need to line up.

How long the est time for queue-ing? is it possible if i arrive at west station at 7.30, to catch train to xian at 8.05? (condition : i haven't got the ticket yet, just booking online)

I have to say that the time is indeed very tight. In total, you have 35min only. If there is a long queue for the collection and security check, you may miss your trip. Usually I think you need to be there at least 1 hour in advance.