In order to the effective control of external devices, the design of load physical simulation platform of control system based on ModBus protocol comes out. The upper computer uses C++ Builder XE2 master communication program design, the Hypogynous Machine uses Siemens S7200 SMART, using the ModBus RTU protocol library design program, the computer connected to the PLC together to form a costeffective load physical simulation platform automatic control system. Through the PC send and receive experimental data verify the effectiveness and feasibility of the design of the physical simulation platform load control system.