The nonlinear resonance problem of composite lattice sandwich panels is studied when both 1:3 internal resonance and external combined resonance occur simultaneously. Firstly, based on Hamiltonian principle and layered displacement field theory, the vibration partial differential equation of composite sandwich panels was established. Then, the ordinary differential equation obtained by Galerkin discretization was solved using multiscale method, and the modulation equation of the sandwich panel was obtained in the presence of both internal and combination resonances. Finally, the bifurcation diagram of the steadystate equilibrium solution of the modulation equation with changes in system parameters was obtained using numerical methods. The effects of different lattice core cell configurations, external excitation frequencies, and external excitation amplitudes on the nonlinear combined resonance characteristics of composite lattice sandwich panels is studied.