This paper analyzes a 500 kV transformer zero-sequence differential protection fault resulted from three-side breakers trip when fault site was out of the reply prevention zone.That balanced coefficient was set defaultly is considered as the main cause of that accident.The paper represents the configuration and action equation of the 500kV transformer zero-sequence differential protection , and puts forward the way which can prevent the faults caused by the configuration of zero balancing coefficient. The way can be adopted by the relay protection personal on site , and is contributed to the safety and reliability of the power system.