May l know the easiest way from TianJin Airport to Beijing Train station ? Bullet train ? How ?

You can take the airport shuttle bus line 1 to reach local railway station. At this railway station, you can take the intercity trains to reach BJ South Railway Station directly.
From the south railway station, you can take the local subway line 4 to reach Xuanwumen Station where you can change to line 2 to reach the local railway station.

Yes,take Bus to the Speed Train Station, there are direct train to Beijing for about 30 minutes.
When you reach Beijing, there are subway to go to city. But beware that there are some stations dont have elevators! You have to carry your luggage up and down stairs when there is changing line. My suggestion is hail a taxi (ask them to use meter or ask the price)If you bring luggage. Enjoy your travel !