The volume grating formed by interfernce of one spherical wave and a planar wave is aperiodic due to its grating vector changes within the volume of hologram. The depth selectivity of aperiodic volume holographic gratings is investigated as spatial-spectral filters in volume holographic imaging systems. The diffractive characteristics of aperiodic volume holographic grating is analyzed by regarding it as an ensemble of local planar gratings cooperating with Kogelnik coupled-wave method. The depth selectivity of an aperiodic and a periodic grating are simulated respectively, and the effect of recording angle on the depth selectivity of aperiodic grating is simulated. Experiments of recording and retrieving volume gratings in photorefractive crystal are designed and implemented in order to verify our simulation results.