According to the definition of satellite’s magnitude, for geosynchronous orbit space satellite, the theoretical computing model of star magnitude was built. Space vectors among sun, observation station and target satellite were derived to calculate the time-space relationship parameters. The simulation curves of magnitude vs. time were drawn based on the model and corresponding parameters. The simulation results were verified to match the experiments ones by using satellite targeting system and astronomical telescope. At last, one geosynchronous satellite with 13.4 magnitudes was observed.