This paper analyzes the factors which affect the drop speed of internal potential of synchronous generators according to the purpose of de-excitation.The processes of de-excitation with different resistors and different approaches are simulated using MATLAB when turbine generator internal short circuit occurs.According to the analysis，the selection criteria of the de-excitation resistor or residual pressure in the de-excitation design of a generator are obtained.That is，the selected de-excitation resistance should make the time constant of the de-excitation circuit a bit less than the bigger one between the time constant of the dq axis damp windings of the generator.At this stage，with the proposed selection criteria，the arc pressure of the DC field breaker required by de-excitation can be in a safe range;and the heat output of the electric arc at the fault point of the stator can be minimized as well.Protection of the generator can thus be real.It also provides significant guidance for modifying the de-excitation standard.