Weather will likely play an important role when you plan your trip to Haikou, and keep in mind that cheaper options are usually available when the weather isn't as good. The hottest months are usually June and July with an average temp of 84°F, while the coldest months are January and February with an average of 69°F. The rainiest months in Haikou are August, October, July, and September, with each month seeing an average of 10 inches of rainfall.