1. A process-orientated design method for machine tool structures, comprising the steps of: (A) defining design conditions, the design conditions including initial configurations for a machine tool, cutting requirements of the machine tool, boundary conditions of the machine tool, a maximum stable cutting depth under a specific speed or a specific speed range of the machine tool and a structural weight of the machine tool; (B) calculating cutting ability, based on the initial configurations and the cutting requirements of Step (A) to generate information of the machine tool that characterizes a relationship between a maximum cutting depth and a spindle speed of the machine tool; (C) performing an optimization, based on the information of Step (B) to determine a frequency range for the optimization; (D) performing a structural topology optimization, based on the frequency range of Step (C) and the initial configurations and the boundary conditions of Step (A) to generate an optimized model; and (E) determining whether the optimized model fits a constraint condition, ending the method if the optimized model of Step (D) fits the constraint condition, repeating Steps (B)˜(D) if the optimized model does not fit the constraint condition of Step (A); wherein the optimized model of the design conditions defined in Step (A) minimizes a structural mass with respect to a steady-state cutting depth.