The laterally nonlinear vibration of axially moving plates was analyzed by the incremental harmonic balance (IHB) method. Firstly, the motion equations of an axially moving plate were derived by Hamilton principle. Then, the Galerkin method was employed to discretize the governing equations, and the IHB method was applied to solve the nonlinear equations. Finally, the Floquet theory and Hsu’s method were used to investigate the stability of the solution. The free vibrations and the forced responses of an axially moving plate with internal resonance were studied. The effects of aspect ratio of plate were also discussed.