The assessment of socioeconomic development is of great significance as it can help governments objectively and comprehensively understand the status of national socioeconomic development and identify gaps, thereby informing policymaking. In this study, three types of data (statistical, geographic, and remote sensing data) were combined to establish a socioeconomic development index (SDI) that considers human living standards, urbanization, and transportation infrastructure to provide a comprehensive assessment of socioeconomic development status. Considering the uneven and rapid development in the countries involved in the Belt and Road Initiative (BRI), these countries were selected as the focus of a case study to validate the new index. SDI was used to classify the level of socioeconomic development in each BRI region, and the performance of each of the three SDI sub-indices for predicting socioeconomic development status was evaluated through linear regression. The results indicate that human development, urbanization, and transportation accessibility showed spatial heterogeneity in the BRI regions, with urbanization index having a particularly heterogeneous distribution. Out of the total BRI area, over one quarter was classified as having a low level of socioeconomic development, approximately 55% had an intermediate level of socioeconomic development, and one-fifth had a high level of socioeconomic development. In three quarters of the BRI, socioeconomic development was mainly restricted by slow modernization. In one-fifth of BRI regions, socioeconomic development was primarily restricted by inadequate transportation infrastructure. Socioeconomic development was mainly limited by low levels of human development in only eight countries. The contributions of human development, urbanization, and transportation accessibility differed among the regions with different levels of socioeconomic development. Human development was the best predictor of socioeconomic development in regions with low levels of socioeconomic development, whereas urbanization and transportation accessibility were more important in regions with relatively high levels of socioeconomic development. Finally, several policy recommendations were suggested based on the above results.