This paper studied the global dynamic properties of a quasi-Hamiltonian vibro-impact system. By referring to Melnikov function for homoclinic orbit in the quasi-Hamiltonian system, the subharmonic Melnikov function for periodic orbit was constructed. A quasi-Hamiltonian vibro-impact system was given to illustrate the application of the procedures, and the validity of the function was verified with numerical results. At the same time, the global bifurcations and coexistence of multiple solutions for this vibro-impact system were analyzed in the improved cell mapping method. The number of coexistent attractor changes with the change of external excitation force, and domains of attraction with complicated fractal boundaries are intertwined.