The three-dimensional intensity pulse function of a confocal scanning microscope for secondharmonic wave is derived and compared with that of a two-photon fluorescent confocal scanning microscope. By use of Fourier transformation,the three-dimensional intensity pulse function is theoretically investigated in frequency domain where the transverse and axial cut-off spatial frequencies,together with the transmission bands are given. The results show the optical resolution for second-harmonic wave is better than that for two-photon fluorescence. Such conclusion is also proved from the uncertainty principle.