Abstract: The backfilling of foundation groove of immersed tunnels is an important procedure in the construction, but the influence mechanism of its seismic performance is seldomly mentioned in the previous researches. In order to investigate the influences of the stiffness of backfill materials on the deformation of flexible joints in the immersed tunnels, the numerical tests of longitudinal dynamic response analysis under incidence of P waves are carried out for Ruyifang immersed tunnel in Guangzhou by using the particle-spring-beam model with implementation of the longitudinal displacement method and the flexible joint model with the longitudinal limit devices. The results show that the numerical model and method can make the structures respond in a timely and reasonable manner. When the longitudinal response displacement method is used for calculation and analysis, the time histories of multi-point displacement in the free field should be extracted as the excitation of the structures. The untreated soft silt clay cannot be used as the backfill materials. If the density of sand or gravel is increased, the seismic capacity will be further improved. The research results may provide a more reliable model and design basis for the response analysis and seismic design of immersed tunnels under earthquakes.