The range gated imaging system combines the technologies of pulsed laser illuminating and range gated ICCD. To measure and evaluate the resolution of range gated imaging system under certain atmosphere conditions, a simulating measurement setup is designed. Two light sources are used for simulating the signal and backscatter light respectively. Based on the method of equivalent irradiance on photo cathode, the methods of power calculation and power range determination for simulating light sources are provided. In the case of image intensifier using S 20 photo cathode coupled with CCD, based on the model of optical backscatter and signal for range gated imaging system, the power of simulating light sources are calculated.