The invention discloses an online strategy optimization calculating method for electric system transient state safety and stability emergency control, and belongs to the technical field of electric system emergency control. According to the online strategy optimization calculating method for the electric system transient state safety and stability emergency control, based on a transient state safety and stability quantitative analysis theory, control performance index calculation methods of direct current system power emergency modulation, generator removal, load removal and partial power grid disconnection are proposed with control measure cost and control effects taken into consideration; according to different characteristics of safety and stability problems of transient state power angles, voltages and frequency, control measures are selected in a candidate control measure space according to control performance indexes from large to small; corresponding active power control measures are further matched according to steady state frequency estimated values after control, calculation and transient state safety and stability quantitative evaluation of the control measures are further carried out, and the control measures are increased continuously through iteration. The online strategy optimization calculating method for the electric system transient state safety and stability emergency control overcomes the defects in an existing online strategy optimization calculating method for the transient state safety and stability emergency control and meets the actual demands of the online strategy optimization calculation for the large power grid transient state safety and stability emergency control.