Abstract: The 6G space-air-ground global networking communication technology is developing rapidly, and high-altitude platform stations (HAPS) and satellite earth stations (ES) reuse multiple communication frequency bands such as C-, Ku-, and Ka band, thereby easily leading to harmful electromagnetic interference due to their proximity deployment. Therefore, conducting research on the quantitative evaluation of interference coordination distance is of great engineering significance. In this paper, the operating characteristics and the interference mechanisms of the two types of systems are analyzed, and a model of the interference to ES due to the uplink and downlink of HAPS is established; moreover, the separation distance is quantitatively solved. The conclusions are drawn as follows. In the 38 GHz~39.5 GHz frequency band, the maximum coordination distances needed from ES to the ground gateway (GW) and the customer premises equipment (CPE) uplink are 600 m and 5 700 m, respectively. For the HAPS-to-GW/CPE downlink and an elevation angle of 90° for ES antenna, the minimum separation distances required reach 4.7 km and 11.8 km, respectively. At an elevation angle of 20°, frequency protection measures must be implemented when the distance between the ES and the HAPS nadir ranges falls into 29.6 km~57.8 km (GW) and 14.4 km~74.8 km (CPE), respectively. At an elevation angle of 10°, the corresponding critical separation distance falls within 106.8 km~109.1 km (GW) and 27.9 km~144.3 km (CPE), respectively. The results of quantitative analysis can provide quantitative support for the collaborative site planning of HAPS and ES.