The invention discloses an optimal design method for an internal structure of a machine tool body. The optimal design method comprises topological optimization of the internal structure of the machine tool body and multi-objective optimization of thickness and height of a ribbed plate of the machine tool body. The topological optimization of the internal structure of the machine tool body comprises the following steps: establishing a body three-dimensional CAD (Computer Aided Design) model; determining a topological optimization region and a non-topological optimization region; establishing a finite element model of the topological optimization; determining boundary conditions; determining optimization objectives and constraint conditions and establishing a finite element model of the topological optimization; the multi-objective optimization of the thickness and the height of the ribbed plate of the machine tool body comprises the following steps: establishing a parameterized finite element model of the body according to the topological optimization result; designing sample points; determining optimization objectives and constraints; performing finite element analysis at the sample points to extract values of the optimization objectives and the constraints at each sample point; establishing agent models of the optimization objectives and the constraints; solving the multi-objective optimization problem. Through the method, the internal structure of the machine tool body is optimized, and the time is saved.