The function of the host computer developed by the high-level programming language in the current production line is undefined and it is difficult to integrate with OPC technology. Therefore, this study combines the operation process of the general production line to design a host computer monitoring system based on OPC technology, which is applied to an actual assembly line. Qt and KEPServerEX are combined to construct an efficient OPC communication mechanism in the system, and the data received from the server is input into modules of storage, real-time display calculation, alarm processing, and chart analysis for further processing. In addition, the system also includes control modules such as station check and marking, enabling the client’s control over the PLC. Experimental results prove that the system has high stability and complete functions, which can meet the requirements of the host computer software in most production lines.