Based on sediment dynamics and jet erosion theory,the paper carried out submerged jet experiment of different target under certain velocity of the erosion of sand bed and fine sand bed.Through capturing the change of scour hole in short time by using high-speed camera,it analyzed the law of this hole depth before 10s and 1s. The first 1 second is separated into 24 time periods to analyze the subtle change.The results show that scour depth is logarithmically before 10s; the trend of the fine rustle bed lags the erosion of sand bed at the beginning of jet scour ;the scour depth demonstrates the trend of first increase and then decrease and exists an optimal target under this velocity.The result has certain reference value for the construction of dredging.