The invention discloses a control performance quantitative evaluation index calculation method for modulating electrical power system direct current emergency power to improve frequency safety, and belongs to the electric power system and automation technology field. The control performance quantitative evaluation index calculation method for modulating the electrical power system direct current emergency power to improve the frequency safety includes steps: calculating transient state frequency safety participation factors of units in a grid according to transient state safety and stability quantitative information; using equivalent mutual admittance module value of a direct current convertor station bus and a electric generator bus in the grid to perform weighted summation on the transient state frequency safety participation factors of the units in the grid; rapidly calculating each electrical power system safety control performance quantitative evaluation index modulated by the direct current emergency power according to specific value between active power change after direct current system emergency power modulation and active power change of a splitting cut set fracture surface. The control performance quantitative evaluation index calculation method for modulating the electrical power system direct current emergency power to improve the frequency safety can rapidly evaluate availability of different direct current emergency power modulation measures when frequency problems occur in an isolated grid after the grid is split, and thereby provides quantitative analysis and decision support for electrical power system direct current emergency power modulation frequency stabilization and optimization control.