The invention relates to an early warning control method and system of rapid steam temperature reduction of a thermal power unit. The method comprises the following steps that (1) an early warning time period is uniformly divided into a plurality of sub time periods, when circulation moment reaches different sub time periods, the maximum steam temperature value in each sub time period is counted,and the maximum steam temperature value in the early warning time period is obtained after high value selection; (2) a steam temperature decreased value in the early warning time period is calculated;(3) an alarm is given out when the steam temperature decreased value in the early warning time period exceeds the alarm limit value, and steam temperature rapid descent protection signals are generated after the quality of measurement points is additionally judged when the steam temperature decreased value in the early warning time period exceeds the early warning shutdown threshold value; and (4) switching value judgment is performed on the different steam temperature fast descent protection signals of the plurality of measurement points, and a tripping protection signal is determined. According to the method and system, the steam temperature rapid reduction can be accurately and effectively judged, the tripping protection signal can be sent out in time, so that the safe and stable operation of the power set is facilitated.