Abstract: As the core component of a navigation system, the stability of satellite clocks plays a crucial role in the positioning, navigation, and timing accuracy of GNSS systems. To comprehensively analyze the long-term performance and stability of satellite clocks in four major GNSS systems, a series of methods were employed, including median analysis, clock deviation fitting models, and Hadamard variance method. After preprocessing the precise clock offset products provided by iGMAS, a comparative analysis of the performance of satellite clocks across different GNSS systems was conducted. The results show that the hydrogen masers in BDS-3 and Galileo exhibit high stability, with stable data trends, few interruptions, and a stability level at the 10⁻¹⁴ magnitude for the millisecond scale and 10⁻¹⁵ for the microsecond scale. The rubidium clocks of BDS-3 and the GPS system perform slightly worse, with some data jumps and interruptions. The millisecond stability of BDS-3 rubidium clocks is at the 10⁻¹⁴ level, while GPS satellite clocks demonstrate millisecond stability at the 10⁻¹³ level, with both systems achieving 10⁻¹⁴ stability at the microsecond scale. GLONASS satellite clocks exhibit poor data continuity, with significant jump phenomena, and both the millisecond and microsecond stability are at the 10⁻¹³ level, showing the overall worst performance.