A new design approach about Offner-type imaging spectrometer is presented based on the relation between the meridianal focusing curve and the sagittal focusing curve. It can be not only applied to the Offner-type spectrometer with the three concentric components,but also the traditional Offner-type spectrometer with two concentric components. What′s more,it can be flexibly optimized based on existing components,which overcomes the complexity and pertinence of the existing methods. The procedures of the tangensy and intersection relations of the meridional and sagittal focusing curres and the design of existing components are shown. An example of optimization design with the existing components is presented. With the relative aperture of f/3.2,the smallest RMS spot radius of 2.3 μm at the wavelength range of 400-1000 nm can be achieved,which is very close to the diffraction limit at the wavelength of 1000 nm. An imaging spectrometer is developed successfully using the above parameters,which can achieve the indexs after preliminary test finally.