In order to study the flow-induced vibration characteristics caused by gas-liquid two-phase flow in a flexible pipe with large deformation, a numerical simulation platform for partitioned fluid-structure coupling under the condition of the internal two-phase flow has been developed. The interface between the gas and liquid phases is tracked based on the volume of fluid method, and the mesh deformation of fluid domain is considered by the arbitrary Lagrangian-Euler (ALE) dynamic mesh technique. Meanwhile, the flexible pipe is established by the finite element method. The fluid domain and structural domain are coupled by adapting the coupling library preCICE. The results indicate that the and three types of mode switching are identified on the basis of the switching characteristics. The vibration of the large-deformation flexible pipe is dominated by the first order and second order oscillation modes. The mode switching mainly depends on the length, recurrence frequency and the axial distribution of liquid slug in the flexible pipe. The large deformation of the pipe promotes the fusion of the short bubble slug and changes the length of the liquid slug significantly, thus affecting the vibration amplitude and flow pattern transition boundary of the pipe.