Abstract: We analyzed the difference between bidirectional filter and other methods of precise orbit determination(POD) for LEO using GPS.The theory of GPS-based orbit determination for LEO using bidirectional filter was deduced from satellite motion equation and LEO GPS zero-differential(ZD) phase observation equation.Two GRACE satellites orbits were computed using the software developed by our institute.We compared results with the orbits provided by JPL and independently validated with k-band ranging(KBR) data.We found that POD using bidirectional filter improves not only the accuracy of individual filter result at the beginning of the data arc evidently,but also the accuracy of orbits in the whole data arc.We also found that the method of GPS-based orbit determination for LEO using bidirectional filter is feasible and the orbits of GRACE computed by the software are at a level of better than 5 cm in radial,transverse and normal direction,respectively.