With the growth of demand and the improvement of process complexity in nuclear power, petrochemical and other industries, the safety monitoring of the status of key equipment lacks direct, real-time and online monitoring methods. It is difficult to achieve accurate diagnosis and early warning of faults. In this article, the shell and tube heat exchanger, key equipment in the petrochemical and nuclear power industries, is selected as the research object. By analyzing the fouling mechanism of the heat exchanger and its impact on the heat transfer performance and applying the total heat transfer coefficient to establish the characterization relationship among the heat transter efficiency, the heat transfer mean temperature differential and the fouling thickness, a virtual sensing method of the fouling of the shell and tube heat exchanger is proposed. The method uses high-precision and easily accessible operation data of the heat exchanger, including temperature, pressure and flow, to realize the online accurate sensing of the fouling thickness of the heat exchanger. Finally, test is implemented by using a high-pressure vessel test piece on the hot state functional test bench circuit. The results show that the virtual sensing model of scale in heat exchanger based on the total heat transfer coefficient can monitor the fouling thickness above 2. 2% of the tube diameter within a 30% error range.