Abstract: In order to study the co-frequency synchronous interference of the broadband terrestrial 5G system to the air-to-ground (ATG) system in the 3.5 GHz frequency band, this paper conducts the inter-system co-existence analysis between the 5G system and the ATG system in this frequency band. By using the technical parameters of the 5G system, we simulate the interference of the 5G system base station to the airborne ATG system and the interference of the 5G system user to the ATG base station in urban, suburban and rural scenarios, and calculate the average interference power density of the 5G system base station to the airborne ATG system and the 5G system user to the ATG base station in different scenarios. The results show that terrestrial 5G system base station has little impact on the co-frequency interference of the airborne ATG system, which can be regarded as acceptable interference. Meanwhile, this paper also gives a reference for the deployment isolation distance of ATG base stations.