An algorithm for precise satellite clock offset estimation based on GPS with the GPS Toolkit (GPSTk) has been developed.Using International GPS Service (IGS) precise orbits and observations of 5 IGS stations,GPS precise clock offsets were estimated at 30-s intervals.Compared to values determined by Center for Orbit Determination in Europe (CODE),the agreement of post clock offset estimates was at the level of about 0.05ns and the real-time 0.33ns; and compared to the ninth-order Lagrange interpolation values,the estimated values showed a great advantage.The clock offset estimates were then applied to Precise Point Positioning (PPP),the RMS of the Up coordinate differences was about 0.05mregarding the known IGS position.The proposed algorithm can be used to PPP within several hundreds kilometers away from the Continuously Operating Reference Stations (CORS) and shows great significance for positioning and gravity surveying in the area where is difficult to set up nearby base stations.