Abstract: To study the influence caused by sampling,here a method,in which the comb function was introduced to sample the fringe patterns,was adopted.Combining with the knowledge of the convolution properties,the phase error caused by sampling in practical composite Fourier-transform propfilometry was discussed along two directions,one is along the phrase variation direction and the other is the vertical direction.The criterions for the selection of sampling frequency are derived respectively along both the directions,and the results indicate that the sampling frequency along the phrase variation direction must be at least four times as the fundamental frequency in order to avoid the spectrum overlapping,and along the orthogonal direction the sampling frequency must be at least three times as the larger frequency of the two carrier frequencies.The computer simulation and experiments verify the theories.