In order to improve the measuring accuracy of blade root bending moment, an automatic calibration process and algorithm for wind turbine blade root stress were proposed. When the wind is light, the wind turbine is controlled to idle without grid connection, and the blades are fixed at different pitch angles, and different bending moments are generated by the blade gravity in the blade root coordinate system, so as to obtain the calibration input. Based on the assumption of linear measurement, the calibration coefficient and central wavelength are obtained by linear regression method. Taking a domestic 7 MW large offshore wind turbine as an example, the simulation results verify that the errors of the selected calibration conditions meet the design requirements. At the same time, the method is applied to the field test data, and the accuracy is verified by the test data. The test results prove the validity of the calibration process and the algorithm.