In order to study shielding characteristics of electromagnetic shielding fabric, a method based on computer image analysis is proposed to build an arrangement model of metal fiber. At firstly, some important parameters about shielding effect are summarized, and then a new algorithm based on threshold segmentation for fabric texture extracting is put forward. By ascertaining an optimization threshold, the fabric image is divided and a feature cluster gray image is established. Furthermore, the feature matrix of metal fiber comprising only two gray values which are threshold value and feature value is founded by using normalized method. At last, calculation formulas about metal fiber content per unit area and equivalent thickness are established and the arrangement model of metal fiber is built. Experiments and analysis show that the method given in this paper is able to describe accurately the arrangement of metal fiber in fabric and it will provide a reference for studying properties of electromagnetic shielding fabric.