Based on Microsoft Visual C++6.0, software of online automatic monitoring system for total phosphorus and total nitrogen in seawater is developed, including the controlling interface of the on-line monitoring system, the driving programs of device, the processing and display of experimental data and the query of historical monitoring data. The software mainly consists of five parts of design process module, manual experiment module, real-time monitoring module, parameter configuration module and serial communication module. Windows graphical interfaces and option card technologies are adopted in the software system, so that this system has friendly visualized man-machine interface and stable operation, and it meets the requirements of experiments.