This paper proposes a load distribution model for the ball screw feed system with the assembly errors of the guide rails, which is used to predict the variation law of the load distributions for the ball screws and carriage-guide rail systems. Firstly, under the action of the contact loads of the ball screws, the bending deformation of the screw axis corresponding to each ball is derived. Secondly, a static model of the machine tool bearing system with the assembly errors is established. Then, the load distribution model of the ball screw feed system is proposed based on the load balance and deformation compatibility equation, and the solution process of the proposed model including judging the ball-raceway contact state is given. In addition, the results are compared with those of the existing models to demonstrate the validity of the proposed model. Finally, the relation between the curve shape of the load distribution and the bending of the screw shaft axis is explored, and the variation law of the load distributions for the feed system under the effects of the axial, radial loads, and assembly errors is discussed.