Abstract: This paper focuses on the physical layer authentication technology based on channel response in satellite-terrestrial integrated communication systems, including research status, challenges and prospects. Firstly, it outlines the security challenges in such open, wide-coverage systems and highlights the advantages of physical layer authentication, including low complexity and fast response. Next, the main categories of physical layer authentication are reviewed, with emphasis on channel response-based mechanisms. A four-step authentication framework of “request-challenge-response-decision” is proposed, which improves authentication performance by optimizing hypothesis testing statistics and deriving optimal decision thresholds. However, this technology faces three major challenges such as the complexity of authentication mechanisms, significant performance degradation under non-ideal channel conditions, and security threats from diverse attack methods. Finally, future research directions are prospected including programmable channel authentication assisted by reconfigurable intelligent surfaces (RIS), quantum-enhanced hypothesis testing to break through classical detection limits, and zero-cost continuous authentication through integrated sensing-communication-computing. These advancements aim to evolve physical layer authentication from passively utilizing channels to actively designing channels, providing lightweight, highly reliable, and scalable security authentication solutions for space-air-ground integrated networks.