Natural lightning is a global phenomenon. If a lightning mapper is onboard a geostationary satellite, it can be used to detect the location, time and strength of the lightning in a global area. Because the function and performance of such a mission are greatly different from those of other optical sensors for earth observation, its instrument should be designed on the basis of new theory and technology. The analysis of the spectral, temporal and spacial characteristics of natural lightning, the understanding of space lightning detection environment and the research on the interaction between natural lightning and its environment are very important for the design optimization of the instrument. For this reason, the design features of a lightning mapper for a geostationary satellite, including signal maximization, background subtraction and onboard data processing, are discussed.