Recently, weak signal tracking is a focal point of research in the field of GNSS receiver design. In this paper, a novel algorithm based on differentially coherent frequency-locked loop (DCFLL) is presented, which is not limited by the navigation bit transition. The integration time of differentially coherent FLL algorithm can be extended as long as 100ms or 200ms. Thus the increasing integration time improves the tracking sensitivity of GNSS receiver. The experimental results show that the proposed algorithm can successfully track GPS signal with received power as low as -160dBm. The selection principle of some key parameters of DCFLL is also introduced.