In order to meet the requirements of ground calibration of star sensor and assorted usage with single star simulators, a light source coupled by integrating sphere and multiple optical fibers is designed based on the characteristics of conventional light source of star simulators. The luminance of each single star emitted by star simulation is controlled by each coupled beam. According to the requirements of indicators, the types of light source, integrating sphere and optical fiber can be confirmed by analyzing the energy in perspective of photometry. Using simulation model established by Light tools and analyzing the simulation results, it is found that the fiber-coupled light source meets the design requirements, and the illumination simulation error of each magnitude is similar to the practical illumination error and both of them are not more than ±10%.