In view of the influence of large deflection and root connecting flange deformation on the bending moment and measuring accuracy of deformation during the static testing of wind turbine blade, calculation formulas of blade deflection and bending moment after deformation are obtained based on the flexibility method and the piecewise rigidization method respectively. Moreover, the influence brought by flange deformation is modified and the simulation model of wind turbine blade under multi-point static loading is established. Then, the theoretical calculation results are compared with the FEM analysis data. The results show that the bending moment error of the section near the blade root can be reduced to 1.07% considering large deflection when designing the loading scheme. After correcting the blade deflection caused by flange deformation, the calculation error of blade deflection at 5.7 m is reduced by 0.1%, so as to ensure the test accuracy of full-size blade under static loading.