Much higher performance of the stator earth fault（SEF） protection is required on the large-sized generator. It inclines to use SEF protection even though generator is at stand-still without excitation in rotor. Therefore， it is a trend to install the SEF protection by sub-harmonic voltage injecting. Two criterions of the SEF protection with voltage injection are improved. Firstly， a safe current restriction is added in the fault resistance criterion for the trip stage. And， the setting method of the safe current restriction is given. Secondly， the effects of the transitive over-voltage on the zero-sequence current criterion are analyzed. The zero-sequence current criterion is set into two stages. One is the high value stage used to fast trip， and another is the low value（sensitive） stage with a block criterion of the main transformer earthing on the HV side. By improving the criterions， there is no trip while the earth fault current is too small even though the fault resistance is zero， so that there is no impact on the power system. And there is no mis-operation caused by the transitive over-voltage. The reliability and the response speed of the SEF protection by sub-harmonic voltage injecting are improved.