A special imaging spectrometer based on a novel combined Prism-Grating-Prism (PGP) is introduced.Its work principle and structure are presented,which consists of a PGP, a front collimated and a rear focusing lens. A holographic volume transmission grating is used in the PGP for the acquirement of high diffraction efficiency and for the convenience of cement and alignment with the prisms. The optimized design result of this PGP imaging spectrometer is reported. Its spectral coverage, resolution, and size respectively are visible light from 400 to 800nm, about 1.6nm, and shorter than 85mm.