Sapindus mukorossi G. has been considered as a potential feedstock for forest-based biodiesel in China. To optimize the cultivation of soapberry and ensure its sustainable supply, an environmental life cycle assessment (LCA) was conducted using a chronological approach combined with extrapolation. Soapberry plantations with two degrees of cultivation intensities were comparatively analyzed. For the studied environmental categories, nitrogen fertilization accounted for half or more of the global warming potential, primary energy demand, acidification, and eutrophication potential. The main contributors to ozone depletion were pesticides and potassium fertilizer. The plantations with a relatively low cultivation intensity presented better environmental performance, mainly due to the lower input of fertilizers, but they are not a priority choice for soapberry cultivation because of low yield. Stakeholders should focus on how to reduce the environmental impacts of the plantations with a relatively high cultivation intensity in this area. Overall, classified management, increasing the yield, reducing the inputs of chemicals, and decreasing the unproductive years are the key ways to improve the environmental performance of soapberry cultivation in Southeast China. Woody biomass carbon should be included in LCAs, and 3.71–5.11 t CO₂ can be fixed by soapberry plantations per ha year, indicating that soapberry cultivation provides a net carbon sink.