Harmonic reducers are generally supported by flexible bearings. The elastic deformation of the flexible bearing enables the harmonic reducer to satisfy high reduction ratio performance. By considering the flexible outer ring and non-circular inner ring of the bearing, a general analytic model was developed to calculate the ball load distribution of flexible bearings. The validity of the proposed analytic model was proved by studying two types of flexible bearing mounted on elliptical cam and four-force action type cam, respectively. Several results validate the use of proposed analytic model to assess the ball load distribution instead of more time consuming FEM. Influences of design parameters in the flexible bearing on the load distribution were investigated, which makes reference for the optimal design of the flexible bearing.