Simulating star magnitude value and its precision is the key for inspecting the detectivity of calibrated optical measuring equipment. The principles of simulating magnitudes of space target scene simulator (STSS) are proposed at first. Based on Planck blackbody radiation law, the causal model of the simulated magnitudes and luminous flux, photon flux in the inner field is deduced, and the major factors that influence the magnitudes value are analyzed. A relative measuring method is used to scale the star magnitudes of STSS on the basis of considering the difficulties of scaling the dark star magnitudes. Afterward, all magnitude error factors, which will arise the simulate magnitude fluctuation, are pointed out. Finally, the theoretic formula of the simulating magnitudes precision error is worked out for the relative measuring method. Experimental results show that the scale ability of STSS achieves 12m magnitudes, and magnitudes error is less than ±0.2m.