We propose a scheme based on the photorefractive effect to manipulate polychromatic spectra. A sinusoidal phase grating can be obtained by exposing photorefractive material to light, and it is used to diffract monochromatic or polychromatic light. The resultant diffracted spatial intensity distribution for monochromatic light is mapped to spectrum distribution for polychromatic light via the spatial-spectral correspondence relationship for mono-polychromatic light diffraction. Theoretical analyses and numerical examples show that for different detection angles, the diffracted spectrum exhibits different features, and that specific spectral line selection or line filter effects can be achieved. We also discuss the possibility of applying this spectral scheme to detect temperature or the grating's period changes.