The EPC of tags has uniqueness, different tags can be distinguished by its binary-tree structure. In the reader's processing area, there are a lot of idle nodes in the binary-tree that is composed of some tags. If the reader ignores them when searching, all tags can be identified with high efficiency. In this paper, its mathematic model of efficiency is analyzed from simplicity to complexity. First of all, the case of two tags with four bits EPC is explained, then the case of two tags with N bits EPC is generalized. Secondly, by converting complexity into simplicity, a general conclusion is obtained based on probability and induction. Its simulating result indicates that its rate of identification is 50%.