A three-phase breaker trip accident caused by failure of single-phase re-closure during transient earth-fault of a 500 kV transmission line is analyzed. The main connection of accident system adopts 3/2 circuit breaker connection， in which the process of single-phase reclosing employs the manners of early and late re-closure. The accident is caused by the transient zero-sequence current resulting from single-phase re-closure and the steady-state zero-sequence current generated by open-phase operation of power system. Improvement measures such as optimizing protection logic， modifying protection setting value and selection of re-closure mode are put forward. Finally， after contrast and analysis， modifying protection setting value is chosen to avoid similar accident， which obtains satisfied effect in practice.