In response to the UN 2030 Agenda for Sustainable Development, this paper proposes a new National Sustainable Development Index (NSDI), based on the modification of the Human Development Index (HDI). The purpose of our research was to improve the widely adopted HDI index by incorporating more comprehensive sustainability perspectives, so as to help policy makers to better analyze the sustainability-related issues facing their countries. After clarifying the concept of sustainable development, our research suggests that this term represents a coordination and configuration of economic, social, and environmental aspects of development, with its major focuses on balancing intra-generational welfare and maximizing the total welfare across generations. We then put forward a novel NSDI framework including 12 indicators from dimensions of economy, resource environment, and society, and calculated the weights of 12 indicators using the entropy method. To further validate our proposed index, this paper also measured the NSDIs of 163 countries in the world, and compared this index with the HDI and other well-known modification indices of HDI. The results showed that the NSDI is a reliable and relative complete index for sustainable development assessment, which makes up for the shortcomings of existing indices.