high speed train from Haikou to Sanya

I would like to take the high speed train from Meilan airport to Sanya. Where can i purchase the tickets and when buyin the tickets,do i have to specify my end destination(sanya) or is it possible to take the train and halfway decide to alight at wanning instead, take a short walk around in wanning before continuing my journey to sanya? can the tickets be bought online?