Abstract: The S7-PLCSIM can be used to examine the running status of the PLC program in the electrical system design process. First of all, eddy current testing process flow was analyzed, secondly, electrical input and output signals address distribution of the PLC was described. Then based on S7-PLCSIM interface descriptions; practical S7-300 PLC was simulated by using S7-PLCSIM. The problems could be found and dealt with timely in the process of simulation. The program development efficiency was improved and the time for debugging was saved. An example showed that the simulation result was the same as the actual PLC operation. The PLC program after simulation test was safe, stable and reliable.