For Global Satellite Navigation System (GNSS), the performance of satellite clock impacts the performance. And for multi-system integrated navigation, the time offsets among GNSS are effected on the level of navigation performance. Nowadays, the development of Galileo space segment is speedily. The 4 Galileo satellites were launched brings the Galileo constellation to 26 satellites, extending the global coverage of the constellation. At present, the number of usable Galileo in-orbit satellites is 17. There are 3 IOV (In- Orbit Validation) satellites and 14 FOC (Full Operational Capability) satellites, except the latest launched satellites. The data in this paper comes from this system. This paper will present a scheme of Galileo in-orbit satellite clock assessment. The evaluation results about frequency stabilities will be detailed shown at paper. On the other hand, The GNSS system time offset monitoring and forecast system based on UTC (NTSC) at National Time Service Center. And next, the system time offsets (especially GGTO) deriving from the monitoring system and Galileo navigation message are analyzed. The results show that the time difference between Galileo System Time (GST) and UTC (NTSC) among satellites are small. And the time offset which derived from each satellite change from -25 ns to -2 ns. The consistency between monitored GGTO and broadcasted GGTO is higher.