Climate resource is one of the most important natural tourism resources in tourism development. The plentiful climate resources not only have the special landscape function, but also have impact on tourism demand. Climate comfort degree effectively responses the comfort of climate in tourism destination for tourist, it is the key factor to tourism development. It directly affects the length of tourism season and annual variation of tourist flows. The climate data of 30 years from nine meteorological stations and tourism data of 5 years from tourism administration of Hainan province were used in this paper. By adopting Comfort Index of W. H. Terjung, correlation and regression analysis, this article analyzed the relationship between climate and annual variation of tourist flows in Hainan province. The results suggested that the advantage of tourism in Hainan was winter and the most comfortable period of tourism began from current November to next March. According to the correlation analysis, climate influences tourist flows remarkably, and comfortable degree of climate is the main factor that influences annual variation of tourist flows and tourism decision to Hainan. Furthermore, air temperature correlates negatively to tourist flows. Compared to Haikou, tourism of Sanya is more sensitive to climate. In view of the importance of climate to tourism in Hainan, it will improve the attractiveness of tourism product when climate factor is integrated into tourism product for better advertisement. At the same time, for the negative correlation between tourist flows and climate, it is easier to forecast tourist flows so that we can make preparation ahead of schedule. Therefore, this study provides important references for tourism planning and investment.