In mechanical load measurement for large wind turbines, the accuracy of load calibration for edgewise and flatwise bending moments implemented by blade gravity has a direct influence on the testing effectiveness for blade and rotor loads. Therefore, analyzing the bending moment analytic results of the blade gravity at the strain gauge measurement position is the key to blade load calibration. Firstly, using the principle of analytical geometry, an bending moment analytic method for blade gravity is proposed based on optimized three-dimensional coordinate system and coordinate transformation. Secondly, in order to eliminate the influence of the main shaft tilt angle on the optimized three-dimensional coordinate system, a method to adjust the direction of the blade gravity vector in the optimized coordinate system is proposed, and the analytical expressions of blade edgewise moment and flatwise moment are obtained taking into account various influential factors. Finally, in the actual blade load test project, the two calibration analysis methods are applied respectively, and the blade load test results obtained by the two methods have good consistency and representativity. The validity and feasibility of the proposed analytic method for gravity load calibration of blade bending moments are verified by theoretical analysis and testing results.