Abstract: This paper puts forward a new concept about "boundary point".The event in normal distribution that greater than thrice standard deviation is used as a small probability event and takes them as the evidence of boundary point for "pick up" and "drop down" in image segmentation.Compared with other methods,it makes decision making computation of "pick up" and "drop down" simplified greatly.The experiments on two pictures of air image show that the segmentation effect of new method is effective and it is a method of image segmentation with great practical.