Rapidly changing phase error correction of interferential fourier transform spectrometer is researched. A complex number is reconstructed, which the real part is the data of zero-crossing single sided interferogram and the imaginary part is the data of short double sided interferogram. After once Fourier transform of the reconstructed complex number, the method can avoid the serious error caused by polynomial fitting of phase error as the phase error is rapidly changing. Then the high precision spectrum can be obtained, based on the characters of real number Fourier transform and discrete Fourier transform. The simulation results show that when phase error can′t change gradually with wave number, conventional Mertz method can′t correct the phase error which is serious, however, the method which is proposed by this paper can correct the phase error, and can gain perfect spectrum. The new method can be widely applied to spectral reducing of interferential Fourier transform spectrometer.