Abstract: On the basis of active imaging model of CW laser and range-gated, it is proved that range-gated technique is an effective method for restraining the influence of back-scattering in terms of single CCD cell SNR(single noise ratio) and image MSE(mean square error). Range-gated technique can enlarge the imaging distance and improve the image quality. Based on the model, some principles to select key parameters of range-gated are figured out. These principles are: (1) the laser pulse width is as short as possible for both increasing the single CCD cell SNR and improving the image quality, (2) the laser duty cycle is as big as possible in a certain power of laser for longer imaging distance, (3) the laser pulse frequeney is as high as possible in some cases.