A line synchronization method in an overlapped spectrum duplex OSD system having K pairs of communications lines. Each pair of communications lines includes an uplink and a downlink. The method includes synchronizing downstream synchronization symbols of downlinks in all of the pairs of communications lines, and separately synchronizing an upstream synchronization symbol of an uplink of each pair of communications lines with a downstream synchronization symbol of a downlink of each pair of communications lines. The method may be applied to crosstalk channel estimation to distinguish NEXT and FEXT crosstalk channels when upstream and downstream spectrums are overlapped.