The problem of ammonium salt crystallization is very important for the safe production of the heat exchange system in hydrogenation plant, which often leads to blockage, corrosion leakage, and unplanned shutdown of the heat exchange system. It is of great significance to the study of the crystallization process and distribution characteristics of ammonium salt. In order to explore the problem of ammonium salt corrosion failure in the high pressure heat exchanger of diesel hydrogenation plant, the ammonium salt crystallization temperature was determined based on the concentration of components, and the C++ ammonium salt crystallization model and user-defined function (UDF) were written. Through isoparametric modeling of the heat exchanger, numerical simulations of the multiphase flow field, heat and mass transfer processes were carried out using ANSYS Fluent finite element analysis software and embedded UDF to analyze the intrinsic causes of corrosion failure. The mechanism of ammonium salt crystallization can be divided into heat transfer control and mass transfer control. The simulated ammonium chloride crystallization corrosion location and the actual corrosion coincided. The significant factors that influence the ammonium salt crystallization can be get through the orthogonal experimental analysis. The independence of the significant factors were analyzed to get the specific impact on the ammonium salt crystallization. Further, it provides a basis for the optimization of process device operation.