Throughout the review of several typical space-borne low-light remote sensing devices onboard the domestic and international satellites, this paper introduces the design principle and components of the Medium Resolution Spectral Imager-Low Light (MERSI-LL) onboard FY-3E. It describes the special three gain combination design of MERSI-LL low-light band (LLB) for the characteristic of large dynamic range of radiance signals on the twilight orbit. The working principle of the MERSI-LL onboard calibrator based on solar diffusing/transmission board is described in detail. In this paper, the selection of three gain channels and normalized fusion processing methods are given, and processing procedures of low-light channel data are introduced. The paper summarizes the several calibration methods for LLB including on-board calibration, vicarious calibration, ground-based artificial light source calibration, and cross calibration. The quantitative scientific product retrieval capability of MERSI-LL is briefly summarized, and the near-constant contrast (NCC) image products generated based on MERSI-LL observation data are displayed. The generation process of typical nighttime light products from LLB is mainly described. Examples of its products and potential applications are given. Finally, based on the lessons of data processing and product generation of Early morning Satellite MERSI-LL LLB, the future improvement ideas and application perspectives of the instrument are discussed.