Abstract: With the rapid deployment and application of low orbit satellite constellations around the world, there is an increasing risk of co-frequency interference of low orbit satellite constellations to geostationary satellite systems. To protect geostationary satellite systems, the current ITU Radio Regulations set equivalent power flux density limits or interference-to-noise ratio limits for non-geostationary satellite constellations, but the applicable frequency bands and limit standards are still being continuously improved. In order to accurately and quantitatively evaluate the co-frequency interference of low orbit satellite constellations, the methods and evolution of the equivalent power flux density limits and interference-to-noise ratio limits are investigated in this work. The analysis of the relationship between the mentioned limits and the practical interference of geostationary satellite systems is presented. Taking a typical inclined orbit constellation and a polar orbit constellation as examples, the effect of different criteria on the determination of interference of low orbit satellite constellations to geostationary satellite systems is analyzed. When the practical interference threshold of a geostationary satellite system is lower than the reference threshold chosen by the ITU in setting the equivalent power flux density limits, it is recommended that low orbit satellite constellations choose the interference to noise ratio limits as the basis for carrying out frequency interference analysis and determination.