A novel kind of Fourier transform spectral imager with high detecting sensitivity and stability is presented. This spectral imager is based on the Sagnac lateral shifting interferometer with one rotary reflecting mirror (therefore named ROSI). Because there is no entrance slit,the throughput is very high and therefore the sensitivity. The datacube of two dimensions of spatial information and spectral information can be obtained by starring mode. Some principle defects of the current high throughput(étendue) spectral imagers are overcomed.