For a type of C prebending machine, in order to test its strength and stiffness requirements and avoid the problem of high safety factor, the model was simplified, and the finite element model was established. However, the preliminary finite element analysis showes that maximum stress 261.17 MPa exceedes the material yield limit requirements and concentrates at the throat of fuselage. In order to further strengthen the strength of prebending machine and avoid the problem of heavy structure, according to the preliminary analysis result, the orthogonal test was set up, and the structure parameters of two different diameters of pull rods, side plates, octagonal stiffener and internal stiffener of upper beam weldment were taken as variables. Then, the parametric modeling was conducted, and the results of sixteen groups of finite element analysis were obtained . Based on the least square method, the regression coefficients and equations of the whole deformation, stress and weight and structural parameters of fuselage were calculated in SPSS, and a mathematical model was established to optimize the target function. Finally, the lightweight parameters of test were calculated by the Matlab optimization toolbox. The results show that the weight and stress of prebending machine reduce 17900 kg and 46.67 MPa respectively, and the lightweight ratio is 7.34%.