A bit synchronization method and system for a GPS, which receives a satellite signal sent by a satellite to accordingly produce a synchronous signal. Each data bit of the satellite signal consists of N CA-codes. A data buffer receives and stores the data bits of the satellite signal. A matched filter filters the N CA-codes of each data bit in order to compute correlations of the N CA-codes of each data bit, and accordingly output a correlation set corresponding to the data bit, wherein the correlation set has N correlations. A selector selects one with the greatest value from the N correlations and uses the CA-code corresponding to the one with the greatest value as the synchronization signal.