We present the optical design of modified Dyson imaging spectrometer (MDIS), to get a fast F# and high resolution ability under the large focal planerelief, ameniscus lens and a plano-aspherical lenswere added into classical Dyson construction consisting of Dyson lens and concave grate. Firstly, MDIS can solve the defects that the detector and slit are hard to be arranged in a classical Dyson imaging spectrometer. Moreover, under the large focal plane reliefconstraintsimposed by assembling and work environment, MDIS is an F/1.8 and spectral resolution 0.45 nmdesign, covering the spectral range 0.38～0.9 μm. Secondly, to our knowledge this is the first time that a fast and high spectral resolution Dyson design is under 12 mm focal plane relief. Although the simplicityof the classical Dyson construction is weakened, MDIS has a very good practical value. Specifically, MDIS can realize the function of a relay imaging system with a 12 mm slit length as well as 250 mm×92 mm×92 mm system size. The design result showed that image quality close to the diffraction limit has been obtained. That is, the value of MTF at all working wavelengths in all fields was greater than 0.7 at 100 lp·mm-1; the maximum of the keystone and the smile at all working wavelengths in all fields were respectively 1 and 0.7 μm. Finally, we can say MDIS provides an effective way to detect atmospheric remotesensing , agriculture, forestry and marine organismsinformation and so on.