Abstract: [Objective] In mechanical load measurement for large-scale wind turbines, main shaft bending moments have less chance to reach a comparable level for tower bending moments, due to the limitation of structure and size of the main shaft. During the load calibration process for main shaft bending moments, moments generated by rotor gravity at the main shaft load measurement section shall be evaluated accurately to measure main shaft tilt and yaw moments with high reliability. [Methods] Based on optimized and improved three-dimensional coordinate systems, an analytic method for main shaft bending moments generated by rotor gravity was proposed, in which the principle of analytic geometry was applied. The contribution of hub gravity on main shaft load measurement section was evaluated as a basis before the focus of blade gravity effect on the same section. To solve the problem of affecting the optimized three-dimensional coordinate system by the presence of main shaft tilt angle, a novel solution was proposed, where the direction of blade gravity vector in the improved coordinate system was adjusted with the same degree as that of the main shaft tilt angle in a specific vertical plane. The gravity vector of a rotating blade in an arbitrary azimuth angle was then decomposed into each axis of the optimized three-dimensional coordinate system. The resulting moment at the section center of main shaft bearing from every gravity component was calculated separately with details before synthesizing results in the same coordinate axis. Bending moment distribution from main shaft bearing center to load measurement section center was obtained by applying moment balance principle for both hub gravity and own gravity of all blades. The combined bending moment contribution by three blades is equivalent to a single mass point with its gravity of the sum of those of three blades. [Results] In a mechanical load test project for some type of wind turbine, the proposed load calibration analysis method was applied, where the results were compared with those by conventional main shaft bending load calibration. The main shaft load results from the proposed method demonstrated representativeness of wind turbine mechanical loads, and coincide with design specifications of wind turbine. [Conclusion] The validity and feasibility of the proposed analytic method for gravity load calibration of main shaft bending moments are verified by theoretical analysis and testing results.