Hi, you can check by filling the booking form in the 'International to Mainland China' column. So far as I know, there are no direct flights but conjoint flights transferring at Guangzhou, Xiamen, Inchon, Kuala Lumpur or Taipei available.

For you reference, you can fly to Beijing first. The flight from Jakarta to Beijing is available on Tuesdays, Wednesdays, Fridays, Saturdays and Sundays.
22:00-07:45, Jakarta to Beijing, Garuda Indonesia, available on Tuesdays, Fridays and Sundays.
01:10-11:05, Jakarta to Beijing, Air China, available on Wednesdays and Saturdays.

Then take a train to Tianjin since Tianjin is close to Beijing. Beijing-Tianjin Intercity Railway has been under operation since last August and it only takes half an hour to cover the distance. You can buy the train tickets upon arrival at the Beijing South Railway Station. There are nearly 60 trains daily, the earliest one is at 06:35 and the last one is at 23:00. The train departs every 10-15 minutes. The first-class ticket fare is CNY69, and the second-class fare is only CNY58. Very convenient.