Hi, i wish go to Suzhou from Shanghai during christmas. How is the train traffic?

Well, as I know, the traffic situation won't become serious during that period of time. Don't worry.
There are many bullet train schedules between these two places every day. So it's ok for you to buy the tickets when you arriving at the train station.
If you wanna book them in advance, you can ask help for TravelChinaGuide. They provides online ticket booking service. Their contact information is bookingtravelchinaguide.com.