The phase error caused by leakage in a 3-D object measurement Fourier transform profilometry (FTP) method is discussed. Because the deformed fringe pattern captured by CCD camera is a function with limited extension in space, it must be extended to infinite space before discrete Fourier transform is computed. If the extended period is not integer times of the grating period, the smooth fringes can not be obtained through extending. When Fourier transform of the discontinuous fringe is calculated, phase error will occur, which is called leakage error. In this paper, the relationship between extended period and frequency leakage is analyzed, the calculation model of phase errors caused by leakage is proposed, quantitative analysis of phase error is presented. A fringe extrapolation method is put forward to decrease the leakage errors. Computer simulations and primary experiments have confirmed the analysis.