Power head is one of the key parts of force transmissibility in core drilling process, its performance affects engineering quality and efficiency of the drilling. According to market requirements, torsion increasing design of power head for the YDX -1200L core drill was introduced. The finite element analysis approach was adopted to simulate the gear case when it was in the first gear with maximum load. On this basis, the thickness of the side wall and fixed dragging plate were chosen as design variables, and multi-objective optimization was used based on ANSYS Workbench, the weight of box body was reduced under the condition of meeting the stress. It provided the theory references for the design of gearbox and the study method would also be a reference for the design of other kinds of case.