An accurate mathematical model for calculating satellite ephemeris is givenin this paper, which is designed to apply to near the Earth and approximatelycircular orbits. The model uses 10 constant parameters to fit the secular and longperiodical variations of the orbit, while the short periodical variations caused bytesseral harmonics of the geopotential field are based on the known theoreticalresults. By sorting the terms with same frequency the formulae become quite simple.In addition, the model is also flexible to use and can be justified with differentaccuracy requirements up to 50 meters.