The underwing landing gear layout referred to the structural form in which the landing gear is installed under the wing. During the load calibration test, the landing gear had a constraint load on the wing root, so the actual load on the wing root differed greatly from the applied calibration load, and the load equation of the wing root could not be accurately established. This paper proposed a load calibration test method which could establish bending moment, shear and torque equations. By measuring the constraint load, solving for the constraint load response coefficient, decoupling the constraint load from the calibration load and reconstructing strain bridge response. This article conducted virtual experiments to verify the method and conducted ground load calibration test and flight test on a real aircraft. The wing root load was successfully measured in the load flight test, which showed that the method was correct and feasible.