EPCglobal_Class1 Gen2 standard employs Q algorithm to adjust frame length automatically in dynamic frame slotted ALOHA tag anti-collision algorithm. The Q algorithm has empty slots, wasting resource timing. Because of the high demand for real-time performance and the large number of tags, in blood management, it is not acceptable to waste time resources. In this paper, a method of pre allocation slots twice is proposed, and the numerical simulation is performed. The results show that the proposed method can significantly improve the efficiency of tag identification.