The invention discloses an out-of-step cross-section optimized selection method for improving the voltage and power angle stability of isolated grid after the out-of-step, belonging to the technical field of power system safety and stability control. In the invention, first, the step-out section is detected in real time; the boundary substation is determined; and then the alternative out-of-step cross-sections are determined according to the coherent constraints; and for each alternative out-of-step cross-section, the damage index of each alternative out-of-step cross-section to the divided power grid structure after the out-of-step during the out-of-step control is determined; and finally, according to the principle of minimum destructive index, the optimal out-of-step cross section is selected. The invention avoids the long-chain power supply structure formed by the divided power grid due to the out-of-step control, and the voltage stability and the power angle stability of the divided power grid can be improved.