The possibility of frequency overlapping was discussed when the Fourier transformation of deformed fringe for discontinuous object that contain mutation was executed by the method of dual-frequency grating.The condition of separation between f1 of low frequency grating and f2 of high frequency grating was deduced,and frequency overlapping of the same grating because of the nonlinearity of detector was analyzed.For the overlapping between f1 and f2 is the chief condition,computer simulation and experiment on the condition prove that f1 and f2 are overlapped when f2<2f1,f1 and f2 are separated when f2>2f1.