Welded metal bellows are the important parts of seal components, whose stiffness has a significant effect on sealing performance. Taking two kinds of DN70 bellows with S-type waveform as the research objects, the finite element model of single-wave bellows was set up, the suitable constraints were established, and the influences of the waveform geometrical sizes of the wave plate with line segments and circular arc segments on the stiffness were studied. The waveform size which has a smaller deviation with the design formula was got by the optimization analysis. The results show that the waveform size has aobvious effect on the stiffness of bellows; The wave plate with inner circle bevel edge waveform has larger stiffness than that with inner circle straight edge waveform; The influence of straight segment waveform on the stiffness is larger than the arc segment waveform.