Abstract: In complex marine environments, submarine pipelines are prone to forming suspended spans. Under the competitive mechanisms of internal and external flow excitations, inter-span action significantly impacts various complex dynamic behaviors, including bifurcation, periodicity, and chaotic motion. To investigate these effects, a vibration model for a double span pipeline was established by simulating pipe-soil constraints with torsion and tension springs. The generalized finite difference method along with the Houbolt method are employed for discretization. The numerical results are then juxtaposed against both theoretical and experimental data to validate the model's efficacy. Based on engineering case studies, the differences in vibration characteristics between single span and double span pipelines on the seabed were analyzed. The results demonstrated that the inter-span action enhanced the pipeline's vibrational amplitude and velocity, leading to maximum vibrational amplitude and energy concentration near the pipe-soil support points in higher-order modes. These effects altered the pipeline's vibrational frequency distribution, causing "frequency locking" and narrowing the outflow velocity range corresponding to odd modes. Inter-span actions increased the non-periodic nature of the pipeline's dynamic behavior, promoted synchronization of vibrations at various spatial points, and even triggered global chaotic motion during modal transitions.