The presence of fouling reduced the heat transfer capability of heat transfer equipments and increased the flow resistance of the medium. Thus the resulting series of economic losses received worldwide attention of the relevant heat transfer industry and countries. For the heating system fouling, direct measurement is nearly impossible. And it is extremely difficulty of structuring mathematic model. Although there are existing monitoring methods, results are not satisfactory. This paper intends to develop a new on-line measuring and monitoring system for heating system fouling. The operating theory of this on-line measuring and monitoring system is based on the soft-sensor technology and Expert System. We select some easily measurements as primary variables, such as pressures, flow rates and temperatures. Through some algorithms, we obtained dozens of secondary variables, for example, the coefficient of flow resistance, the efficiency and cost of heating system and so on. Based on these variables, we construct the knowledge base of this System. This system mainly uses Delphi and Excel as development tools. Now, the system is running well in some heating station, and has reached the expecting result.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.