Compared with traditional point-scanning laser radar, the imaging laser radar based on range gating and gain-modulation ranging principles has faster ranging speed, but at the same time, signal noise ratio (SNR) of single-frame image is lower. According to principles of imaging laser radar, the ranging accuracy model under the influence of shot noise is built. Based on the model and the characteristics of airborne imaging laser radar, a new method which employs the techniques of gray image registration and superposition to improve ranging accuracy is put forward. Then the factors, such as the flight attitude, light uniformity and superposition frame number that influence the application of gray registration and superposition, are analyzed theoretically. The results show that flight attitude has no effect on the method and the light uniformity′s effect is small when the uniformity is better than 40%. There will not be other errors between the targets when superposition frame number is within a certain range. A ground of dynamic experiments and aerial experiment are conducted to verify the validity of the method.