Abstract: Carbon steel pipe is widely used in condenser of PWR nuclear power unit. Its corrosion during outage can give rise to prolonged downtime. Furthermore, the corrosion products migration to steam generator accumulates sludge formation. It can not only cause water chemistry environmental degradation, reduce heat pipe heat transfer efficiency, but also increase the corrosion risk of steam generator. In order to study the adsorption mechanism of octadecylamine (ODA) on carbon steel surface of inner wall of main tube of PWR condenser, molecular dynamics simulation software was used to simulate the adsorption process of ODA on carbon steel surface. From the perspective of thermodynamics, calculating the adsorption free energy of filmforming amine molecules on carbon steel surface by thermodynamic data and determining the type of force field constituting the adsorption free energy are effective methods to study the adsorption of organic molecules. In the first step, the adsorption model of ODA on metal interface was established by using amorphous cell module. According to the strongest X-ray diffraction peak, the Fe (110) plane of preferred orientation of metal Fe was selected to construct the interface adsorption model. In the second step, sorption module was used to calculate the layout, and ODA and H2O adsorption simulation was realized. The COMPASS Ⅱ force field, atom based summation method and NVT ensemble were used to filling 20 ODA molecules and 100 H2O molecules into the super cell vacuum layer ranging from 5% to 90%. The distribution model of ODA and H2O molecules in the super cell structure was preliminarily obtained. In the third step, the Forcite module was used to calculate the stable adsorption configuration, and the Gibbs free energy before and after the adsorption was calculated to obtain the adsorption free energy. In the fourth step, the adsorption procedures according to the above steps were repeated layer by layer until the fifth layer, and the adsorption free energy was also calculated layer by layer. The main conclusions are listed as follows. The absolute value of adsorption energy gradually decreases with the increase of ODA adsorption layers indicating that the trend of ODA spontaneous adsorption gradually weakened. The adsorption energy of the first layer is mainly generated by van der Waals force accounting for more than 98%. The second layer is mainly generated by electrostatic force accounting for more than 62%, and the rest is generated by van der Waals force and other nonbonding forces. The the third layer is mainly generated by electrostatic force and other nonbonding forces accounting for more than 88%, and the rest is generated by van der Waals force. The fourth layer is mainly generated by electrostatic force accounting for more than 80%, and the rest is generated by van der Waals force and other nonbonding forces. The fifth layer is mainly generated by electrostatic force accounting for more than 69%, and the rest is generated by van der Waals force and other nonbonding forces. It can be seen that ODA physical adsorption takes place on carbon steel surface at 40 ℃, and the adsorption energy is generated by van der Waals force, electrostatic force and other nonbonding forces. Van der Waals force is the main force in the first layer, and electrostatic force is the main force in the other layers. According to the research results of this project, the physical adsorption mechanism is proposed. ODA takes place multilayer physical adsorption on the surface of carbon steel, and the number of adsorption layers increases with the increase of ODA concentration. The adsorption energy of the first layer of ODA molecule is mainly provided by van der Waals force, and that of the other layers of ODA molecule is mainly provided by electrostatic force. The spontaneous adsorption tendency of ODA decreases gradually with the increase of the adsorption layer.