Small targets in infrared images are usually detected by using the relative characteristics of small targets and backgrounds. Compared with conventional descriptors, detectability of infrared small targets (DIST) is a better metric of the relative characteristics of small targets and backgrounds. However, the quantification method of DIST cannot describe the situation of darker targets in brighter background, which is common due to the intense reflection of sunlight. Moreover, its value is not normalized, so that it is not well suited to evaluate the degree of difference between small targets and backgrounds. An improved quantification method of DIST is proposed to address these problems, and a detecting method is developed based on the improved DIST. The experimental results show the validity of the improved quantification approach of DIST.