Based on anisotropic diffraction of volume grating in the photorefractive LiNbO3 crystal, an optical wavelength demultiplexing (WdeM) scheme implemented by a single volume grating is theoretically discussed. By only one grating written by two He-Ne laser beams at a specific wavenumber, the scheme can realize wavelength demultiplexing around telecommunication wavelength 1550 nm, and the theoretical possible wavelength suitable for WDM is from 1400 nm to 1650 nm. For anisotropic diffraction, the polarization of diffracted and readout beams are orthogonal to each other, which can largely improve the signal-to-noise ratio. Besides, only one grating written in the crystal can avoid time-writing complexity in the multiple grating writing.