To address the development design of level extension for mining the No. 3-1 coal seam at Level II in Cuncaita Coal Mine, Based on constraints including geological conditions, existing development layout, and production system connectivity, research was carried out on the design of the second-level extension and production system optimization. First, the current state of the mine development layout and production system was analyzed from the aspects of industrial site arrangement, development method, transportation system, ventilation system, and drainage system. Second, the design of the second-level extension was determined by analyzing the extension development method, level division and elevation, main roadway layout, panel division, and layout of the first working face. Finally, the production system for the second level was optimized in terms of main(auxiliary) transportation system, ventilation system, and drainage system, achieving effective integration with the mine’ s existing production system. The results show that through reasonable extension development layout and scientific production system optimization, the second-level extension design of Cuncaita Coal Mine effectively improves the utilization rate of existing production systems and facilities, avoids impacts on mine production during extension construction, and can provide a scientific reference for level extension design in mines with subhorizontal coal seam groups.