In the Internet of Things (IoT) environment, the combination of traditional blockchain technology and Public Key Infrastructure (PKI) faces issues of low efficiency and slow response. This paper proposes an innovative PKI identity authentication scheme based on Directed Acyclic Graph (DAG) blockchain to meet the fast response needs of users and low-computational devices. By integrating the Directed Acyclic Block Graph (DABG) framework with PKI, the scheme optimizes the authentication process for security and efficiency. It introduces a trust voting mechanism for anchoring identity information on the chain and classifies nodes to construct an efficient tree-like logical network topology. Theoretical analysis ensures the scheme's security and reliability. Experimental simulations confirm its advantages in latency, communication redundancy, and throughput, proving its effectiveness in rapid identity authentication response.