The invention provides an on-line three-phase load unbalance processing method suitable for a low-voltage distribution network. The on-line three-phase load unbalance processing method comprises the following steps of: monitoring three-phase load by three-phase load monitoring units on the low voltage side of a distribution transformer, and transmitting monitoring data to three-phase load unbalance monitoring units of the distribution transformer; judging whether the three-phase load unbalance exceeds a standard value or not by the three-phase load unbalance monitoring units of the distribution transformer; if the three-phase load unbalance exceeds the standard value, transmitting signals to an on-line load phase modulation control unit by a communication unit; judging whether the three-phase load unbalance of the distribution transformer is caused by the three-phase load unbalance of a main low-voltage circuit or not by the on-line load phase modulation control unit; and if so, setting an on-line load phase modulation strategy according to the monitoring data of the load monitoring units of branches of three phases of the main low-voltage circuit, sending a control command to an on-line load phase modulation switch combination unit by an on-line load phase modulation judging and controlling unit, and switching the load phases of the branches of users so as to realize the on-line three-phase load unbalance processing.