Spatial heterodyne spectroscopy (SHS) is an imageable spectroscopic technique with high sensitivity, and it can obtain high spectral resolution. According to these features, a line-by-line calculation method is presented for SHS observations, which is fit for the horizontal sounding of water vapor without other gas absorptions. Absorption coefficients of all lines are computed with Voigt line function, and the halfwidths are also modified to the actual pressure. Average transmittance calculation is done by taking account of line continua, and the density of water vapor can be calculated. The real-test data obtained by FTIR spectrometer covering a range from 1590 to 1610 cm-1 is used to calculate the transmittance. It is compared with the result obtained from Modtran computation, and the method is proved feasible.