A clock synchronization method, apparatus, and system are provided according to the embodiments of the present invention. The method includes: receiving time information TM1 from a central office side, in which the time information TM1 is obtained by the central office side by reading a local clock of the central office side when transmission data at a first designated location starts to be transferred, and the local clock of the central office side is synchronous with a Global Positioning System (GPS) clock or a Building Integrated Timing Supply (BITS) clock; determining whether transmission data at a second designated location starts to be received, if so, reading a local clock of the Customer Premises Equipment side to obtain time information TS1; and synchronizing a clock frequency of the Customer Premises Equipment side to the central office side according to the time information TM1 and the time information TS1.