The root of a wing-mounted undercarriage load was investigated in light of how difficult it currently is to test, combined with a certain type of aircraft load test project. A wing root strain-gage bridge was designed that included moment, shear and torque. Considering the condition that there was no load-carrying structure as a constraint point in the outfield, a self-balanced model of the whole structure with multiple-point coordinated loading was established. Using these methods, reliable calibration data was obtained and analyzed with multiple regression, and the load calibration equation was established. In the check condition, the error of the load calibration equation was obtained to be less than 5% in the case of large load and the engineering utilization requirements have been met. The aircraft ran test flights, so the measured load of the wing was derived using the calibration equation. This method can be applied generally to the root of wing-mounted undercarriage load tests in the outfield.