How can I get from port of Tianjin Xingang to Beijing by fast train in the morning and return the same day in the afternoon?

You can take bus 102 from the port to Tanggu railway station and then transfer to bus 621 to Tianjin Railway Station. There you can find bullet train to Beijing South. It takes about 30 minutes by the train and costs about CNY69/first class seat and CNY58/second class seat. It operates from 06:25 to 22:45 on a frequent interval everyday.
You have no need to book it if it is not the peak travel season.