The invention provides a Si-Ge-Si semiconductor structure with double graded junctions, comprising a substrate, a transition layer or an insulating layer formed above the substrate and a strain SiGe layer formed above the transition layer or the insulating layer, wherein the Ge component at the central part of the strain SiGe layer is highest, and the Ge components at the upper surface and the lower surface thereof are lowest, and the Ge components from the central part to the upper surface and the lower surface thereof are in graded distribution. In the invention, the graded junctions are utilized to replace abrupt junctions, thereby forming a triangular hole potential well, and thus, not only hole carriers can be mostly distributed in a high-Ge material layer, but also the problem that the carrier mobility is lowered because of interface scattering can be solved, and the performance of devices is further improved.