Abstract: For the three types of GNSS system time offset monitoring approach，we analyze the basic principles and methods and made a comparison of their accuracy，reliability and implementation，and finding that the PPS-based method is more suitable for time offset monitoring. We present a detailed discussion and description，including hardware configuration，data collection and post processing content for the PPS-based method. In an experiment we use both GPS and GLONASS dual-mode receivers to generate the 1 PPS of GPS and GLONASS second pulse signals，and used the Stanford SR620 time interval counter for PPS precision measurement. We thereby obtained GPS and GLONASS system time offset measurement values，and adopted a FIR filter with a Harming window to get the GPS and GLONASS system time offset average 317. 1 ns，with a standard deviation of 5. O1 ns.