The nonlinear response characteristics of a crack beam subjected to a high-frequency acoustic wave simultaneously with a low-frequency vibration excitation were discussed. A finite element model of the crack beam was developed to perform the investigation, in which a closing crack model, fully open or fully closed, was employed to represent the damaged element. Based on the concept of nonlinear output frequency response functions (NOFRFs), the occurrence of the nonlinear response of a cracked beam subjected to high-low frequencies co-excitation was explained. The dependences of the nonlinear response amplitudes on the crack depth and position were analyzed by using the finite element model to simulate the time domain response of the crack beam. The results show that the high-low frequency excitation method is suitable for crack detection based on the nonlinear effect.