According to the responsivity of photoelectric device for a light source, the formula of spectral matching factor of photocathode for object is deduced. The spectral matching factors of S₂₅photocathodes for dark green paint and green vegetation are calculated and compared. To S₂₅₁, the spectral matching factors for dark green paint and green vegetation are 0.5512 and 0.2783 under the full moon radiation. They are 0.2662 and 0.1255 under the clear star radiation. To S₂₅₂, the former are 0.7174 and 0.6820 and the latter are 0.5741 and 0.4840. It is referential to evaluate visual range of night vision system through analyzing the results.