With respect to time synchronization of LEO (Low Earth Orbit) satellite platform in TTC (Tracking, Telemetry and Command), a traditional model has presented to calculate the time difference from satellite to the ground. By analyzing the error sources that are six in the traditional model, the traditional model has modified to be a new GPS (Global Positioning System) time synchronization model to calculate the time difference from satellite to GPS with telemetry information originated from the satellite. Validation in TTC has shown that the GPS (Global Positioning System) time synchronization model has an error source decreasing to two, and has a time error within 1ms and a time precision less than 1ms, and is applicable to long-term management for on-orbit satellite in TTC.