The invention discloses a thermal power generating unit primary frequency modulation performance assessment index calculation method and system. The method comprises the following steps: calculating an additional weight coefficient of an integral electric quantity according to related electric quantity information of the system; According to the additional weight coefficient, calculating the integral electric quantity of the primary frequency modulation actual increase output of the primary frequency modulation unit and the integral electric quantity of the primary frequency modulation theoretical increase output; And calculating a primary frequency modulation performance assessment index value of the set based on the integral electric quantity of the primary frequency modulation actual increase output and the integral electric quantity of the primary frequency modulation theory increase output. According to the method, the primary frequency modulation performance of the thermal powergenerating unit is examined after power disturbance fault disturbance of the system, a grid-connected power plant can be stimulated to improve the primary frequency modulation capability of the unit,rapid change of frequency is inhibited, and rapid recovery of supporting frequency is supported.