Abstract: The air-conditioning system on a certain ship was controlled by a multi-piece microcomputer. Due to its complex configuration, outdated technology and inconvenient supply of spare parts, the control system should be replaced functionally. Programmable Logic Controller (PLC) is widely used in control systems because of its better capabilities and lower price. Naturally Siemens S7-200 PLC was selected as the air-conditioning contr rol center. Two major factors should be considered during the refit: one was that the hardware substitute must be selected among the COTs and homemade products, the other was that the software program must ensure that t air-conditioning unit could run safely, meet the demands and has fault diagnosis and analysis. The test proves that the refitted system can run very smoothly and fully meet the demands.