Train from Haikou to Sanya - Information Please

We arrive at the Haikou Airport at 5:10 PM on June 29th and will train to Sanya that evening, can we practically make the 6:20 or 6:40 PM trains to Sanya from Haikou or are we looking more at the after 7:00 PM trains. I would like to book our tickets before hand or is that needed, can we expect to walk up to the counter and book our train that day with no problem just depending how fast we get out of the airport and how fast the taxi can get us there? Thanks for any sage advice you can give. Doug