Subgrade connecting has become an important research project in high-speed railway construction engineering. Combining the hub project of Hefei railway, numerical calculation was executed with different subgrade heights to existing line’s safety operation and subgrade stability by FLAC software. It shows that deformations in the vertical direction of the connection between the new subgrade and the old and the old subgrade surface increase following the increment of subgrade height, and the velocity of deformation increment also aggrandize. Reduction of subgrade height is beneficial to trains safety on existing line and subgrade integration property. It will be instructive to control differentia deformation for the connecting subgrade projects in high-speed railway.