I want to be connected with a train from Guangzhou to Fuzhou on 20TH March 2015 after 6pm

I'm afraid not. No direct train service is available for this route. If you don't mind, transfer to another flight to FZ directly. There are several choices after 18:00. If you wish to take a train, I'd suggest you transfer in Shenzhen and the last train to Shenzhen leaves from Guangzhou at 23:08, so you can surely catch one.