The invention provides an evaluation method and system for short-term voltage stability of a power grid. When a power grid operation state monitored in real time by a power grid relay breaks down, a monitoring bus voltage, a load bus current and a real-time relative power angle detected in real time by PMU measurement of the power grid are substituted into a real-time power grid parameter identification equation which is built in advance, and calculation is carried out to obtain load margin; if the load margin is larger than a preset short-term voltage stability value, a voltage of the power grid is in a stable state, otherwise, the voltage of the power grid is in a breakdown state; and the PMU measurement achieves rapid evaluation and margin calculation of the short-term voltage stabilityof the power grid, and enhances on-line monitoring of a short-term voltage stability point by the power grid.