Based on the filtering principle of photorefractive multiple volume gratings, superposition of eight volume gratings at the angle 0.037 degree between the grating wave vectors in the LiNbO_3∶Fe crystal with size of 10×10×10 mm3 has been obtained by using incremental recording method. Those gratings have approximately the same diffraction efficiency. So the application of multiple volume gratings in wavelength demultiplexing system is demonstrated by the experiment. Furthermore, a double wavelength filter using the LiNbO_3∶Fe crystal has been made.Two light beams with the wavelength of 635 nm and 650 nm illuminating the volume gratings in the same direction have been separated by the filter successfully. According to the Bragg condition, the volume grating is used to measure the wavelength of light wave, and the measurement accuracy can reach the order of 10 pm.