Abstract: The modes and applications of identity authentication in common used E-Procurement System (EPS) are analyzed in this paper firstly. Then, the dynamic identity authentication policy model is acquired according to Policy, Protection, Detection, and Response (P2DR) model and architecture. Finally, the identity authentication policy model based on P2DR is proposed for the security requirement of EPS. Considering all kinds of factors such as policy, cost, working scope and so on, this policy model can adjust the modes of EPS dynamically and realize the homeostasis among "security, cost and efficiency" for different security requirements and users in each stage of e-procurement.