Abstract: The interference of China's non-geostationary satellite orbit (NGSO) constellation system to geostationary satellite orbit (GSO) system with fixed-satellite service in Ka band is studied in this paper. Based on combing the ITU-R recommendation and radio regulation, the problem of NGSO constellation interfering GSO system in the different link scenarios are discussed via theoretical analysis and simulation calculation. The simulation results show that the NGSO constellation system would cause serious interference to the GSO system in the overlapping band of both the uplink and downlink. Finally, the measures and suggestions for reducing the interference from the NGSO constellation system are given. This paper give a reference for the interference analysis of the future NGSO constellation system to the GSO system in Ka band.