In January 2012, a certain Pumped Storage Power Station once had a serious accident. The Generator Circuit Breaker (GCB) had the open-phase fault when the generator ran with low load. The relevant protection failed to remove the fault and led to expand the accident scope. Through the comparative analysis of the inverse time negative sequence current protection and the phase angle difference protection of the zero sequence fundamental voltages between GCB both sides, the comprehensive protection based on the zero sequence voltage and the negative sequence current is put forward for the open-phase fault. Considering the phase and amplitude features of the zero sequence fundamental voltages between GCB both sides, new criterions of the GCB open-phase protection based on the zero sequence voltage amplitude ratio and phasor difference are given. GCB open-phase protection is further optimized by these new protection criterions and can be applied in the relevant power plants. This work is supported by Science and Technology Project of China Southern Power Grid (No. K-ST2014-003).