The transformation of zero phase into small phase in vibroseis seismic data processing is a problem of controversy: Is it a necessary process? when will is be conducted ? What is its influence on seismic wavelet? The authors analyzed small phase transformations of the autocorrelation wavelet sweep signal of vibroseis, the simulated earth absorbed wavelet and the practical vibroseis data wavelet. The authors have found that phase drift phenomenon is inevitable in the phase transformation process,and propose the modification method,which can to some extent solve the problem of the phase-shift. The authors affirmed the necessity of phase transformation,and put forward the optimal order to conduct the processing.