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Mangroves serve many important ecological functions and consequently represent a dominant coastal ecosystem.However,coastal regions are very susceptible to ecological damage due to their high population density,urban expansion being one of the most important influencing factors.Accordingly,it is vital to ascertain how urban expansion endangers mangrove ecosystems.This study used the decision-tree classification method based on classification and regression tree (CART)algorithm to extract areas of mangrove and built-up land from Landsat images.A correlation analysis was performed between the change in the area of mangroves and the change in the area of built-up land at the cell scale.This study aimed to reveal the magnitude of the influence of urban expansion on mangrove forests in different periods and in different regions,and to identify the places that are seriously affected by urban expansion.The results demonstrate that this approach can be used to quantitatively analyze the impact of urban expansion on mangrove forests,and show that larger areas of mangrove were affected by urban expansion in the past 30 years.The effects of urban expansion were stronger over time,with approximately 12％ of cells containing mangroves showing a negative correlation between the increase in the area of built-up land and the change in the area of mangrove forests to different degrees from 2005 to 2015.The same quantitative analysis was also carried out in three subregions of Guangdong Province,namely west Guangdong Province,the Pearl River Delta,and east Guangdong Province.It was found that the situations in these three regions were very different due to discrepancies in the distribution of mangroves,the rate of urban expansion,and the awareness of the local govment regarding environmental protection.These results can assist in the management of coastal cities and the protection of mangrove ecosystems.