As the main auxiliary equipment of power plants, the air preheater inevitably generates expansion and displacement during operation, which poses potential hazards to the safe operation of the unit. Therefore, under the premise of obtaining the temperature field using the difference method, the finite element analysis was conducted on the preheater during the start-up process to obtain the stress and displacement fields of the rotor and shell of the preheater. The results show that the expansion displacement of the outer ring of the rotor is different and the maximum expansion displacement occurs at the rotor position at the circumferential flue gas outlet. The maximum deformation of the top vertex of the rotor at this position is about 36.65 mm and that of the lower vertex of the rotor is approximately 26.86 mm. By analyzing the radial seal rubbing, it is determined that the cold end radial seal at the outlet position of the circumferential flue gas side near the central cylinder may collide. During shutdown maintenance or operation, the appropriately adjustion of the operating position of the fan plate can avoid rubbing and reduce radial air leakage.