There are quite many options for you to transfer from Tianjin to Beijing, please check the followings to select the one that matches your requirments best.
Option 1: most convenient:
Passenger can take the airport bus from Tianjin Airport to Beijing Bawangfen Long Distance Bus Station (which is very close to the downtown) directly. Buses depart at 10:00am, 1:00pm, 2:00pm, 3:40pm and 5:00pm respectively Monday to Sunday. Ticket price is RMB 35 Yuan and time duration is about 2 hours.
You can book the airport bus ticket by calling either 22-24905966 or 22-60533328 (please add country code 0086 for international call).
Option 2: most frequently:
Traveling on tight schedule & tight budget? Taking the bullet train to Beijing from Tianjin (East) Railway Station is a good solution for you. Passenger should take the airport bus to Tianjin (East) Railway Station firstly and then change a bullet train there for Beijing South Railway Station. Airport buses depart for Tianjin (East) Railway Station every half hour from 6:00am to 7:30pm Monday to Sunday. Place to take the airport bus is at No. 2 gate, domestic exit of Airport Terminal. Ticket price is RMB 10 Yuan.
There are many trains running from Tianjin to Beijing, check the following link for latest train timetable and ticket price information from Tianjin to Beijing.
Option 3: most comfortable:
You may just want a comfortable trip and not caring too much for money. Well, we would suggest you charter a taxi. Distance from Tianjin Airport to Beijing downtown is about 200km and time duration is 2 hours and taxi fare would be RMB 500-600 Yuan.