The superlarge diameter metal O-ring used for the sealing of reactor pressure vessel will inevitably produce deformation after processing,and the ideal roundness is difficult to guarantee which leads to complex deformation in the process of assembly preloading.In order to study the deformation of elliptical metal O-ring in pretightening process,an elliptical ring model was constructed in the same circumference mode for simulation.According to the contact relationship between the elliptical metal O-ring and the inner or outer walls of the flange,the deformation and displacement of the key section and the change of the motion path of the key section characteristic points were studied.The overall deformation of elliptical metal O-ring was analyzed comprehensively.The simulation results show that the long axis of the elliptical metal O-ring moves in and the short axis moves out along the radial direction during the compression process,which makes the result of overall deformation tend to be round.The section of the long axis and the short axis twists in opposite directions,resulting in torsion deformation of the elliptical metal O-ring.