The multiple-scattering theory based on Monte Carlo (MC) method is described in detail to calculate two typical multiple-scattering instances. With atmospheric absorption coefficient in 1.2 km-1,scattering coefficient in 1.5 km-1 and for a 1 W source, the signal energy is close to 0.27 nW at the distances of 50 m away and it decreases in exponential decay with increasing distance, when the detector and emission are in opposite directions; the signal energy is 0.23 nW at distances of 200 m away and it increases greatly with increasing distance,when the visual angle of the detector is limited by obstacles. Accordingly, a solution is presented to simulate the light propagation when the detector and emission do not have mutual scattering-substance. Experimental results show proposed simulation is correct and it is very practicable in complicated conditions.