Based on the measured topographic data for many years,and combining with conditions of water and sediment upstream of the Yalu river，this paper analyzes the cause of large erosion and deposion on the riverbed of highway bridge waters in the period of bridge construction.The results show that from March 2010 to September 2010，the runoff is larger and the riverbed erosion is obvious,which is featured with erosion at the right side of the protruding shore beach and deposition at the left side of the concave bank deep grooves.The main reason for the large erosion during construction of the bridge is the strong erosion at the early stage and automatic re-balance of the riverbed.During the bridge construction,the dense pile groups of the construction trestle and transport facilities promote greatly the backsiltation after the flood in 2010.