The invention discloses a rapid predicting method for the sand content of a reservoir-exiting sand peak of the Three Gorges reservoir in the flood period. The rapid predicting method comprises the steps that firstly, a reservoir tail main stream control hydrometric station is selected as a sand peak reservoir-entering station, and a nearest main stream control hydrometric station in the downstreamposition of a dam is selected as a sand peak reservoir-exiting station; secondly, the average daily sand peak sand content S of the sand peak reservoir-entering station, the flood detention reservoir capacity V corresponding to the average daily in-front-of-dam water level on the day when a sand peak appears in the sand peak reservoir-entering station, and the average daily flow Q of the sand peak reservoir-exiting station on the day when the sand peak appears in the sand peak reservoir-entering station are collected, and a supposed value of the average daily flow Q of the sand peak reservoir-exiting station on the sand peak reservoir-exiting day is given in advance according to the actual condition; thirdly, the sand peak reservoir-exiting ratio S/S of the reservoir is calculated according to the formula (please see the specifications for the formula); and fourthly, the sand content S of the reservoir-exiting sand peak of the reservoir is calculated according to S/S and S. According to the rapid predicting method of the sand content of the reservoir-exiting sand peak, the river way terrain, the water and sand reservoir-entering and reservoir-exiting process, the in-front-of-dam water level process and a water and sand transportation mathematical model are not needed, required information is less and easy to obtain, calculation is easy and convenient, and the accuracy is high.