The low light level (LLL) night vision system now plays a more important role in military affairs, and people always focus it on the detecting distance. This thesis analyzed the diatance detecting equation, and brought forward a new way to increase the detecting distance suitable for low speed object. This method is multiplying the frame integral time. Based on the revised apparent distance detecting equation, the affect of the frame accumulation to the detecting distance was studied. The influence of the frame accumulation on the detecting distance at different illumination intensities was analyzed theoretically. At about 10-4 lx and 10-3 lx, it can raise the distance to 157% and 131%, respectively. The field experiment at weak illumination showed that this method was effective.