Pls advise how to go from Beijing to tianjin binhai international airport.

Well, here is a free way just carried out.

Go to Beijing South Railway Station to take a bullet train to TJ Railway Station (there are about 80 departures per day).
Upon arrival, go out from north exit and then go underground to take TJ subway line 2 to TJ airport.
Upon arrival, go through customer and security check, then head for the Air Railway Co-transport Counter to apply for reimbursement of the train ticket and subway ticket. So keep them well. If the subway ticket has been recycled, go get an invoice from the staff at the subway station.

Please note that you need to buy railway ticket and subway ticket all by yourself.