As the core spatiotemporal reference infrastructure for the safe operation of civil aviation, the performance of the Global Navigation Satellite System (GNSS) directly affects the safety of high-precision applications and service integrity during critical flight phases. Building an efficient GNSS Monitoring and Assessment System (GMAS) is a key means to ensure the reliability of civil aviation navigation services and achieve real-time anomaly alarms; and scientific site selection is the cornerstone to ensure that GMAS provides credible data support for civil aviation safety. The core functions of GMAS for civil aviation safety is focused on, the theoretical basis and key considerations for the site selection of civil aviation monitoring stations are systematically expounded, and a comprehensive evaluation of candidate sites in typical civil aviation scenarios based on simulation analysis are conducted.