Superposed volume Bragg gratings (SVBG) has the characteristics of excellent wave vector selectivity and adjustable diffraction efficiency. Based on Kogelnik′s coupled wave theory, the potential applications of SVBG for harmonic-wave separation and beam sampling are analyzed. The results show that the SVBG is made up of two pieces of reflecting volume Bragg grating (VBG) and one piece of transmitting VBG. The two pieces of reflecting VBG can highly diffract fundamental frequency and second harmonic beams (harmonic-wave separation ratios are 0.2% and 0.3% respectively), while do little effects on the third harmonic beam (transmittance is up to nearly 100%) in order to make the harmonic-wave separation. The transmitting VBG has low diffraction efficiency for third harmonic beam (only 0.98%). The spatial and temporal intensity distributions of the diffractive beam are similar to those of the third harmonic beam, so the harmonic-wave separation and beam sampling can be realized simultaneously based on the SVBG. The SVBG is a good potential substitution for the traditional grating for the harmonic-wave separation and beam sampling.