With the rapid development of the low-altitude economy in China， the low-altitude airspace is characterized by massive device connectivity and intensive spectrum utilization， posing significant challenges to the real-time safety supervision of unmanned aerial vehicles （UAVs）. According to China’s mandatory standards and civil aviation regulations， UAVs are required to continuously broadcast their Remote identification （Remote ID） information for monitoring and identification. However， the absence of source authentication in standard broadcast protocols introduces security risks. Moreover， existing research lacks theoretical capacity analysis and quantitative evaluation specifically for the Chinese standard broadcast format. To address these issues， this paper proposes a trustworthy construction method for broadcast Remote ID utilizing the national SM2 cryptographic algorithm. By appending digital signatures to standard messages， this method ensures resilient authentication and eliminates potential security vulnerabilities inherent in international algorithms. Furthermore， we formulate a channel capacity model for the Wi-Fi Beacon broadcast system. Simulation results show that the carrier sense multiple access （CSMA） mechanism achieves an 85% performance improvement compared to the pure ALOHA protocol. Under ideal channel assumptions， using 2.4 GHz single-band， 20 MHz bandwidth， 1 s update cycle， and 18 dBm transmission power， the theoretical capacity of the trustworthy broadcast with SM2 signatures is 82 aircraft/km²， which effectively meets the current high-density capacity demand of approximately 15—22 aircraft/km². Additionally， a dynamic signature frequency strategy is developed to balance security and capacity. The proposed signing method and capacity analysis model provide a theoretical foundation and design reference for the future deployment of low-altitude regulatory systems.