In the Fourier transform infrared spectrometers the interferograms are usually sampled by the means of the single-side beyond zero path difference and the interferograms are asymmetric. These phase-corrected technologies are researched and improved in the present paper. In the Mertz method which requires apodization to be higher, the interferograms are apodized by these asymmetric windows so that the width of the main lobe increases and spectral resolution reduced. In order to solve these problems, a method which combines the Mertz and Forman methods is presented in the paper. In the method a double-side interferogram is constructed by the mirrored processing of the single-side interferogram beyond zero path difference, followed by the symmetric apodization, fast Fourier transform (FFT) to rebuild spectrum. And in the method high resolution phase spectrum is computed by the symmetrization of double-side interfergram to improve the phase-correction accuracy. In the experiment, these asymmetric window functions have the lower resolution compared with these symmetric window functions. And the spectrum which is acquired by the improved phase corrected method is compared with the spectra which are acquired by the Mertz and Forman methods, and the improved method has less error than Mertz, less computation than Forman, and the spectral resolution achieved 2 cm-1.