This paper studies PLC control system, adopts computer simulation technology, designs simulation model which can imitate manifold common PLC systems, programs common PLC system simulation application which adopts process oriented program and object oriented programming using manifold tools such as database, SQL(structure query language), Visual C++6.0 etc. This application can read PLC program that made by software, then explain and run step by step according to the user input, and then simulate common PLC systems running status.