The loosely coupled inductive power transfer system is an effective and safe method of power transmission，which uses electromagnetic induction technology to achieve non-contact transmission of energy while solving the shortcoming of traditional power supply with wires.The nonlinear bifurcation phenomenon of the full-bridge resonant converter is analyzed with the change of the coupling coefficient of the loosely coupled transformer.The state equations and nonlinear models of the system are built based on the mutual inductance modeling.The nonlinear behaviors are numerically analyzed when the system resonates，and the complex three-dimensional bifurcation diagram is obtained with the coupling coefficient as the bifurcation parameter.The complex bifurcation behaviors of the system are validated by circuit simulation.Moreover，the border collision phenomenon and the characteristics of the time and frequency domains are analyzed.This study expands nonlinear research on the switching converter to the wireless power transfer system，providing important guidance for reliability design of the wireless power transfer system.