With the rapid development of China's economy and society and the continuous growth of traffic volume, early built highways have begun to show poor traffic capacity and low service levels. In order to solve this problem, China has actively undertaken the reconstruction and expansion of expressways in recent years. At present, many expressway reconstruction and expansion projects have been completed and put into operation, and remarkable results have been achieved. However, the reconstruction and expansion of expressways is different from that of new highways. This process involves issues such as splicing new and old roadbeds and coordinating deformations. In particular, the differential settlement resulting from the splicing of new and old roadbeds poses a significant threat to the roadbed and pavement structure. Therefore, scholars at home and abroad are committed to reducing the differential settlement of new and old roadbeds, and have explored many new ideas and technologies. This paper reviews and combs the existing research results from three aspects: the settlement theory of differential settlement, the influencing factors and the treatment measures of subgrade splicing. It is found that the measures to reduce the differential settlement of new and old subgrade, especially for the arrangement of reinforced structure, the research on the internal drainage structure of subgrade is still relatively lacking. Therefore, it is recommended to conduct further research in these areas, comprehensively understanding the mechanisms and influencing factors of differential settlement in subgrades, and proposing splicing measures suitable for diverse working conditions. This endeavor aims to provide theoretical and practical guidance for practical engineering projects.