By analyzing the influence of background clutter on infrared small targets detection, we have researched a background clutter description method for infrared images of small targets. Three conditions of a background clutter description method are proposed: consistent with subjective judgment; suituble for different infrared images; assistant with improvement of target detection algorithms. Four features are synthesized by considering the relation between targets and backgrounds, and a novel method is proposed for describing the background clutter quantitatively in infrared images of small targets. Firstly, supporting vector machine is emploged to classify the backgrounds. Secondly, the weight of each feature is calculated by analytic hierarchic process for each class of background images. Then the quantification problem is solved. Compared with the conventional methods, the validity of the proposed method has been demonstrated by the experimental results.