Over the years, various mobile communication technologies have been developed and operated simultaneously, which made the mobile communication networks evolved from single-mode access to complex heterogeneous integration. The current 5 G has already accommodated diverse terminals through multiple access paths, but the upcoming 6 G ambitiously aims to achieve ubiquitous connectivity through space-air-ground-sea integrated networks. However, traditional authentication and management protocols, such as EPS-AKA and 5G-AKA, are designed for specific networks and lack the flexibility to adapt to the diverse and dynamic requirements of 6G. This limitation will inevitably result in complex management, enormous overhead, and unmanageable security risks. In this paper, we present an atomic functional module-based 6G authentication protocol combination framework (AotmAuth) to decompose the existing authentication protocols into reusable basic modules, and by combining these modules, flexible authentication protocols can be constructed to meet specific security and performance requirements. The proposed approach can significantly improve the robustness, extensibility and dependability of protocols cobmination, by simplify protocol design, enhance adaptability across diverse scenarios and facilitate quick improvements by replacing or adjusting specific modules. To validate the effectiveness of the proposed solution, we design and develop a 6G heterogeneous access security testbed. The experimental results show that the proposed framework can achieve higher authentication efficiency while ensuring security compared to traditional authentication methods.