Heat exchanger is an important part of fumigation oven. However, in many cases, the actual design of fumigation oven is depended on experience and experimental verification after the design. So, how to simplify the design of fumigation oven has been an important issue. Based on fluent software platform, this research explored the heat exchange mechanism of U pipe heat exchanger by the simulation of U pipe heat exchanger. By considering the structure characteristics of U pipe, a layer of U pipe is used as a basic unit. The basic unit is simulated and the data get in the simulation is numerical fitted Then a function of inlet temperature of U pipe, the layer number of U pipe and the temperature of outlet is get, and a temperature decreasing mathematical model of the layer number and the temperature of outlet is built. The function can reflect the trend of the temperature decreasing efficiently, so it provide a theoretical reference for actual design.