Aiming at the problem of high magnitude target simulation in the lab, a weak-light single star simulator (WLSSS) consisting of light source, adjustable diaphragm, integrating sphere, photoelectric detector and collimator is developed to measure the magnitude detecting ability of detecting cameras and star sensors. The relationship between the readings of photoelectric detector, the spectral radiance in the outlet of integrating sphere and tunable obscuration ratio is deduced. Combining the magnitude definition formula and the image illuminance equation formula, the operation principle of weak-light single star simulator is introduced, and the calibration challenge of high magnitude target is solved. The magnitude simulation range and precision of WLSSS are theoretically analyzed, and results show that the highest simulating magnitude is 19.5 Mv, and the precision is 11.6%. When the simulation magnitude is lower than 15 Mv, its precision is better than 8%. Experiment results show that in the lab, the maximum relative error between simulated magnitudes and KLL-04 wide-range illuminator tested magnitudes is 7.09%. The relative error of measured magnitude detecting ability between laboratory and astro-observation detecting cameras is 1.9% in 6.5 Mv, and 2.6% in 15.2 Mv. The designed WLSSS can effectively simulate high magnitude targets. © 2017, Chinese Lasers Press. All right reserved.