Abstract: The causes, control methods and standards for uncoordinated deformation between new and old subgrades in highway reconstruction and extension projects are analyzed. According to the typical test section of Shijiazhuang to Cixian (demarcation line between Hebei Province and Henan Province) reconstruction and extension project of Beijing-Hongkong- Macao Highway, a finite element model for subgrade widening of highways is established. The additional settlement and horizontal displacement of ground surface as well as transverse slope ratio of pavement are analyzed during the construction period of new subgrade and after construction of 15 years. The geogrid-reinforced technology for the new subgrade is used in this project. The geogrid modulus, reinforced layers and reinforcement length are changed during calculation. The effects of these parameters on surface settlement of the subgrade and transverse slope ratio of pavement are studied. Based on the computational results, the optimization design technology of geogrid-reinforced subgrade widening for highways is presented. The test results can be used as a reference for future studies and design of geogrid-reinforced subgrade in highway reconstruction and extension projects.