In response to the security challenges of Massive Machine-Type Communications (mMTC) and cross-slice handover in 5G network slicing, this paper proposes a lightweight group authentication and key agreement protocol based on a hierarchical architecture. The protocol constructs a collaborative model of user layer, slice layer and operator layer, and integrates advanced cryptography technologies and dynamic security mechanisms to achieve the following core objectives: bidirectional authentication and dynamic key management through layered identity verification and temporary key negotiation to ensure mutual trust between users and slices, with timestamp and random number obfuscation technologies introduced to resist replay attacks and key leakage risks, supporting real-time update of slice keys and dynamic generation of session keys; privacy protection and anti-attack capabilities by adopting anonymization technologies and encrypted transmission to protect user identities and slice sensitive information, and building a security protection system against man-in-the-middle attacks and data tampering based on elliptic curve cryptography integrated encryption scheme combined with hash verification and access control; efficiency and lightweight design through group authentication aggregation and resource optimization scheduling to reduce device computation and communication overhead, with the protocol supporting millisecond-level cross-slice handover response to meet the real-time and high-density access requirements of scenarios such as industrial IoT and smart healthcare. Theoretical analysis and simulation verification show that while ensuring dynamic security of 5G slicing, the scheme provides an efficient and scalable security solution for heterogeneous network environments through the complementary design of hierarchical architecture and cryptography technologies.