A line synchronization method in an OSD system, applied to an overlapped spectrum duplex, OSD, system, wherein the OSD system comprises K pairs of communications lines, and each pair of communications lines comprises an uplink and a downlink; and the method comprises: synchronizing (601, 701) downstream synchronization symbols of downlinks in all of the communications lines; and separately synchronizing (602, 702) 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; wherein the synchronizing (601, 701) downstream synchronization symbols of downlinks in all of the communications lines specifically comprises: configuring all central office sending units that are in a vectoring group, so that downstream synchronization symbols sent by all of the central office sending units are at a same time location, wherein the vectoring group comprises the multiple central office sending units and multiple far-end sending units, each of the central office sending units controls one of the downlinks in the OSD system, and each of the far-end sending units controls one of the uplinks in the OSD system; and wherein the separately synchronizing (602, 702) 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 specifically comprises: configuring all of the far-end sending units that are in the vectoring group, so that upstream synchronization symbols sent by all of the far-end sending units and the downstream synchronization symbols sent by all of the central office sending units are at a same time location. synchronizing (601, 701) downstream synchronization symbols of downlinks in all of the communications lines; and separately synchronizing (602, 702) 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; wherein the synchronizing (601, 701) downstream synchronization symbols of downlinks in all of the communications lines specifically comprises: configuring all central office sending units that are in a vectoring group, so that downstream synchronization symbols sent by all of the central office sending units are at a same time location, wherein the vectoring group comprises the multiple central office sending units and multiple far-end sending units, each of the central office sending units controls one of the downlinks in the OSD system, and each of the far-end sending units controls one of the uplinks in the OSD system; and wherein the separately synchronizing (602, 702) 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 specifically comprises: configuring all of the far-end sending units that are in the vectoring group, so that upstream synchronization symbols sent by all of the far-end sending units and the downstream synchronization symbols sent by all of the central office sending units are at a same time location. configuring all central office sending units that are in a vectoring group, so that downstream synchronization symbols sent by all of the central office sending units are at a same time location, wherein the vectoring group comprises the multiple central office sending units and multiple far-end sending units, each of the central office sending units controls one of the downlinks in the OSD system, and each of the far-end sending units controls one of the uplinks in the OSD system; and wherein the separately synchronizing (602, 702) 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 specifically comprises: configuring all of the far-end sending units that are in the vectoring group, so that upstream synchronization symbols sent by all of the far-end sending units and the downstream synchronization symbols sent by all of the central office sending units are at a same time location. configuring all of the far-end sending units that are in the vectoring group, so that upstream synchronization symbols sent by all of the far-end sending units and the downstream synchronization symbols sent by all of the central office sending units are at a same time location.