This paper uses the model of transient flow to analyse the mechanism of fluctuating pressure propagation within the cracks of bed rock and the cause of fluctuating uplift generation in details. A predicting formula for the maximum fluctuating uplift on rock blocks in a scour pool is deduced. To confirm the theoretical results, fluctuating uplift on different sized rock blocks at the bottom of scour pool downstream of the Three Gorges spillway model are measured.