To avoid the shortcoming of magnification change with wavelength, which will cause the spectral image pixel registration error and provide inaccurate relative spectral signal strength in the classical diffractive optic imaging spectrometer, a novel hybrid refraction/diffraction telecentric optical system combining a diffractive lens with an achromatic lens system is presented, and the working principle is then established. Furthermore, the concrete design example for visible/near-infrared imaging spectrometer is analyzed and evaluated with optical design software Zemax. The results prove the proposed method is valid and feasible. The shortcoming is overcome successfully, and the difficulty of the manufacturing process of the diffractive lens is also reduced. The theoretical and practical guidance is thereby provided for the study, design and engineering of the new diffractive optic imaging spectrometer with constant magnification.