A digital image generating method based on the CCD and optical parameters of sensor system was presented. According to the relation of photometry and radiometry, a model of star imaging gray value was produced to calculate the imaging intensity for target in focal plane. Then, the number and magnitude of stars in the space were counted to obtain the statistic regularity of stars in the field of view by star map software SKYMAP. Finally, the digital image generating method was set up acoording to the statistic regularity of stars in the space and star image characteristic on CCD focal plane.The digital image generating method was simulated based on CCD and optical system parameter, results show that the method can generate a star map with higher apparent magnitude (magnitude＞10) and can satisfy the requirements of space object detection and surveillance for star map.