Abstract: A positioning algorithm based on three satellites is studied in assisted global positioning system (AGPS) in order to realize positioning efficiently in weak signal. AGPS positioning system structure is introduced to improve the performance of receiver and the system provides auxiliary data through general packet radio service (GPRS). When only three satellites are captured, pseudo-range is disposed using fuzzy border disposal technology. The position is located through solving a group of equations with Gauss-Newton algorithm. At last, the positioning result based on different height is analyzed according to real measured data. The mean horizontal positioning bias is less 16m when the bias of height is 10m.