In the design process of hardware-in-the-loop scene simulator of a guidance system, in order to establish an accurate basis for the selecting of the pixel-mapping-ratio (PMR), the influence of PMR on the simulation has been analyzed based on the sampling properties of the detector of unit under test and the spatial light modulator of simulator. The unique characteristic of the spectral distribution of the random targets has been introduced into the simulation model to calculate the modulation transfer function. Furthermore, the modulation transfer function curves at different PMR and phases have been obtained. The results show that if the PMR is either 0.31 or 0.71, the influence of the phase variation on the modulation transfer function is negligible, and the modulation transfer function′s value shows minimum attenuation and good stability. The quantitative criterion can be provided in the design of scene simulators.