The rejuvenation of non-geostationary orbit (NGSO) satellite communication holds the promise of seamless and ubiquitous broadband access from the space. However, the NGSO constellations must share the scarce radio spectrum resources with geostationary orbit (GSO) satellite systems, which results in dynamically changing and unevenly distributed interference to GSO systems. In this context, the ultra-large-scale NGSO constellation incurs a more complicated interference environment with GSO systems, which raises urgent demands on inter-system interference evaluation. In this case, we investigate the inter-system downlink interference from a NGSO satellite mega-constellation to a GSO earth station. Specifically, we consider the scenario where the NGSO and GSO earth stations are co-located, and apply a novel visibility analysis method in the interference modeling to reduce computation redundancy. The interference evaluation is then performed through comprehensive simulations, in which the Starlink constellation with more than 4000 satellites is examined for the first time. The simulation results demonstrate various states of interference on the GSO earth station at different deployment locations. It reveals that the number of visible satellites could influence the angle between the main lobe directions of NGSO satellites and the GSO earth station antenna, which further affects the interference level.