In the precision light screen velocity measuring system, a signal processing technology based on the arithmetic of image processing is put forward to eliminate the interference of random noise to opto-electronic detected signals, so the interval time of the object flying through the two light screens can be gained accurately. It converted the counting of interval time into the counting of end position of the object in the image. At first, the image defects such as isolated point, burr etc. were eliminated to get the image which has only one seriate area, and then, the image area was divided by the average width to get the end position of the object. Each time when the object fly through the two light screens can be measured accurately by using the end position of the object in the image, and the interval time can be gained by subtracting the two time. The experiment show that the system can gain the interval time accurately by applying this processing technology, the speed data is stable and effective, and the system measurement precision can be up to 0.05%.