The invention provides a control system and method for three-phase active power unbalance of a low-voltage distribution network. The system comprises a low-voltage distribution network distribution control device, a metering automation system and a fast synchronous switching switch respectively connected with the low-voltage distribution network distribution control device. The method comprises the following steps: monitoring three-phase active power of a low-voltage distribution network through an intelligent electric meter in a metering automation system in real time; calculating the unbalance degree of the three-phase active power according to the three-phase active power by the low-voltage distribution network distribution control device; if the unbalance of the three-phase active power exceeds the preset threshold, sending a switching command to the fast synchronous switching switch of the corresponding load. The invention directly utilizes the active power in the existing metering automation system, compares and analyzes the three-phase active power, and utilizes the communication network of the metering automation system to control the fast synchronous switching switch to switch the phase. The application utilizes the existing equipment and the control calculation method is simple, thereby greatly saving the equipment investment cost.