Low-altitude networks are fundamental for the informatization, digitalization, and intelligence of the low-altitude economy. Under current policy guidance, domestic operators are actively developing low-altitude network infrastructure. However, the propagation models for air-to-ground channels differ significantly from those for terrestrial communication channels. This is particularly relevant for low-altitude unmanned aerial vehicle (UAV) users, whose uplink transmissions can interfere with ground users, impacting the stability and quality of ground communication. To assess the interference of 5G low-altitude communication at the 2.1 GHz frequency band on ground systems, this paper employs a novel air-to-ground antenna deployment across three stations with nine sectors. A testing method for air-to-ground interference is proposed, utilizing dedicated resource blocks (RBs) for ground cells to monitor the interference caused by low-altitude communication. Data analysis presents two methods: one evaluates instantaneous interference by assessing the duration of interference from UAVs to neighboring ground cells, while the other assesses average interference through the elevation of background noise at ground base stations within a defined coverage area. The findings indicate that during the initial deployment of the 2.1 GHz low-altitude network, air-to-ground interference remains within acceptable limits. This study provides a reliable assessment of uplink interference for 5 G air-to-ground networks, offering significant reference value for the deployment of 2.1 GHz low-altitude networks.