## 5 Conclusions  
From the perspective of the sustainable development of tropical cities, based on ecological quality assessment, a new method of simulating the spatial growth of tropical cities based on ecological constraints was developed in this study.  
According to the evaluation results obtained using the RSEI, the ecological quality background of Sanya is relatively good, but it decreased significantly from 2014 to 2018, which was mainly due to the development and construction of the key development areas in Sanya. Based on the urban spatial development trend from 2014 to 2018, the growth of the urban built-up land in 2030 was simulated. The results show that this scenario leads to the extensive occupation of ecological and agricultural space and is unsustainable. It is necessary to establish an RSEI-based ecological bottom-line pattern as a rigid boundary for urban development.  
Simulation of future urban expansion areas and UGB delineation based on the RSEI-PLUS model can better balance the conflict between ecological protection and urban expansion. Under the premise of ecological land protection, the urban space can also be rationally configured according to the local conditions, and it also accurately reflects the urban spatial expansion under the background of territory spatial planning. Therefore, the RSEI-PLUS-based approach to simulating urban spatial growth is an innovative, highly feasible approach that can effectively support the current territorial spatial planning.  
Due to the uniqueness of natural resources in tropical areas, regional cities are facing problems in the sustainable development of the current economic development, population growth, and urban expansion, as well as the environmental carrying and protection. The results of this study provide solutions for the socio-economic and ecologically sustainable development of tropical cities.  
Conceptualization, N.H. and P.J.; methodology, N.H.; data curation, K.H., M.Y. and Y.Z.; writing--original draft, N.H. and K.H.; writing--review and editing, N.H., K.H. and P.J.; funding acquisition, P.J. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the National Social Science Foundation of China, No. 21 XGL019, and the Hainan Provincial Natural Science Foundation of China, No. 421 RC1034.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data presented in this study are openly available in Geospatial Data Cloud ([[https://www.gscloud.cn/](https://www.gscloud.cn/)]([https://www.gscloud.cn/](https://www.gscloud.cn/)), (accessed on 24 October 2020)), National Geomatics Center of China ([[https://www.ngcc.cn/ngcc/](https://www.ngcc.cn/ngcc/)]([https://www.ngcc.cn/ngcc/](https://www.ngcc.cn/ngcc/)), (accessed on 21 June 2021)), Resource and Environmental Science and Data Center ([[https://www.resdc.cn/](https://www.resdc.cn/)]([https://www.resdc.cn/](https://www.resdc.cn/)), (accessed on 25 June 2021)) and Statistical Yearbook of Sanya City.  
**Acknowledgments:** The authors gratefully acknowledge the two anonymous reviewers for their valuable comments and suggestions, which strengthened the quality of the paper substantially.  
**Conflicts of Interest:** The authors declare no conflict of interest.