Space technology is an effective approach to monitor the status of World Natural Heritage sites. East Rennell Island has been inscribed on the List of World Heritage in danger since 2013. The site includes approximately 37,000 hectare (ha) and a marine area extending three nautical miles to sea. Deforestation and natural disasters have become the increasing factors threatening its sustainable development. Based on the analyses of multi-source long-time series remote sensing data like MODIS and Worldview data, the forest cover change and its future trends in Rennell Island from 2000 to 2020 have been mapped and assessed using Sen + Mann-Kendall and Hurst index models. A land cover classification system derived from the high-resolution Worldview images was developed as a baseline product for monitoring and analyzing future forest cover changes in East Rennell Island. Our results show that: (1) Area of the vegetation degradation is basically same as that of vegetation improvement from 2000 to 2020. (2) The trend of forest cover change is weak in continuity, and significant improvements in damaged vegetation can be implemented but it needs enough protection measures and financial input. (3) This heritage site has a strong ability to regenerate vegetation, and it is recommended to restrict the human activities like the mining, logging, and road construction, which could greatly disturb the unique ecosystem.