In order to improve the tracking sensitivity of the Beidou receiver and solve the problem that the data bits and NH code phase changes limit coherent integration time, a weak signal tracking algorithm based on coherent integration is proposed. Firstly, an optimized bit synchronization/frame synchronization algorithm is designed with bits matching in groups to realize coherent integration within bits. Then, the maximum likelihood estimation method is used to estimate bit reversions to realize coherent integration over bits. Finally, software receiver simulation is carried out. Correlation tests of 20ms and 40ms are realized and the signal-to-noise ratio increase about 16dB. The simulation results show that the bit synchronization method is able to achieve stable and reliable signal synchronization. With bit synchronizing and NH code solving, it can realize coherent integration within or across bits, and effectively improve the tracking sensitivity of the Beidou weak signal.