In order to preliminarily predict the fatigue life of a weld seam, the road load data of a test vehicle were collected. The chassis was dealt with as flexible and a rigid-flexible coupling model was built in the LMS Virtual Lab. Based on the technology of time waveform replication, the virtual iteration method was applied to obtain the vertical displacement driving signal on the target. The driving signal was used as the input of the rigid-flexible multi-body dynamical model of the vehicle to decompose the dynamical load spectra at connection points. Based on the theory of linear cumulative damage and the Volvo method, the fatigue life of the seam weld was predicted by virtue of the dynamical load spectrum. The weld fatigue life is increased by nearly 4 times by optimizing the connection form of the third beam and longitudinal beam.