For the stress concentration regularly occured in the flexible supporting structures for large-aperture mirrors, a new method that composed the NiTi Shape Memory Alloy (SMA) around the flexible slots of the supporting structures was proposed,by which the tensile stress caused by pre-strain of the SMA can reduce the stress concentration greatly without decreasing the flexibility of the structure. Firstly, the stress distribution equation for the flexible supporting structure was established, and the dangerous cross-sections were analyzed. Then, the one-dimensional constitutive equation of the NiTi alloy wires was derived, and two composition schemes, cross-type layout and U-type layout, were designed accordingly.The finite element analysis was applied to the two schemes, and the results show that the U-type composition is superior to the cross-type one in both peak stress and structural flexibility.A parameterized optimization was applied to the U-type layout by taking the tension F of NiTi alloy wires as design variable and the maximum stress p and the distortion δ as optimization objectives.The results demonstrate that the optimum solution is obtained when F is 200 N,which shows that the p is reduced from 76.0 MPa to 37.9 MPa while δ from 0.031 mm to 0.028 mm slightly, and the maximum stress occurs away from the flexible slots. Random vibration tests were conducted for the U-type composition scheme, and the results verify the effectiveness of this method for the stress compensation.