A time synchronization method and system, and a network device, where the method includes receiving, by a second network device, a first packet from a first network device, where the first packet includes a first service frame and a first time stamp (T1), the first service frame carries a system clock of a radio equipment control (REC), obtaining, the system clock of the REC according to the first packet, sending, according to the system clock of the REC, the first service frame extracted from the first packet to a radio equipment (RE), recording a second time stamp (T2) at which the first service frame is sent, and determining, by the second network device, a transmission delay (T) according to T=T2−T1. Hence, a delay between the REC and the RE remains unchanged.