In recent years, tourism has been playing an increasingly prominent role in China's economic development, especially in the tourism-oriented cities. Therefore, Sanya has been selected for research and analysis in this paper to reveal the tourism development law of Sanya. Based on gray system GM (1,1) and seasonal cross-trend model, this article analyzes domestic annual and monthly tourist numbers in Sanya, reveals their development rules, and forecasts their future development trends with this model.