For the airfare from Bangkok to Haikou, you can contact some booking agencies in Bangkok. And there you may get more preferential price.

Besides, on the airlines official webpage, there must listed the contact No. for its branch office in Thailand, So, you can get the number by visiting that page

Dear Mr Meng. I have visited the web site of Hainan Air and as far as i can see, it does not list a telephone number for Hainan Air in Thailand. Please can you help me?
I forgot to mention that i do not live any where near Bangkok,and have been to all the local travel agents in my area, they say that they have no listing for Hainan Airlines, may be you could supply me with an agents number so i could call them direct? I hope you can help me and 'thank-you' in anticipation. Paul. Nr Ban Saray,Thailand.