Abstract: In this paper, design of constellation in need of continuous coverage for ground target is studied. The equations of two variables, gap time of satellite coverage and gap time of plane coverage, which having essential influence on the coverage capability are inferred. They are calculated in the case of constellations with various deployments. Finally, the inferences are validated using the engineer simulation software STK. This paper provides theoretical guidance for constellation design through theoretical analysis, calculated examples as well as engineering simulation.