Space-based scanning tomographic limb imaging spectrometer is demanded urgently by space atmospheric remote sensing. Based on the research objective of spaced-based scanning tomographic limb imaging spectrometer with wide horizontal covering range and minor volume and mass, An optical system of space-based scanning tomographic limb imaging spectrometer is designed using one dimension scanning mirror, off-axis parabolic telescope and aspheric grating spectral imaging system. The field of view of hyspectral imager is 0.75°×0.017°, relative-aperture is 1/1.8, and working waveband is from 280 nm to 800 nm. Ray tracing, optimization are performed by CODE V software. The root mean square (RMS) spot diameter for different wavelengths is less than 34 mm for aspheric grating spectral imaging system. The spectral resolution is 0.84 nm, which satisfies the requirement of specification (≤1.2 nm). The modulaiton transfer function (MTF) for different wavelengths of imaging spectrometer is more than 0.57 at characteristic frequency in the spatial direction. The design results satisfy the requirements of imaging quality, which is suitable for the application of space-based remote sensing.