Ships passing through estuarine locks will cause salt water to sneak into the lock chamber and then invade the freshwater channel,causing water quality deterioration.Previous researches focused on the invasion of high concentration salt water (concentration 20‰-30‰) in sea locks,but the invasion process and characters of low concentration salt water intrusion (concentration 2.23‰-4.17‰) in estuarine locks are still unclear.Based on the large-scale water tank experiments,this paper studies the motion characteristics low concentration salt water and discusses the evolution process of low concentration salt water motion form,the relationship between salinity distribution and motion form,the motion velocity of salt water head,and the relationship between density Froude number and concentration.The results show that,driven by the density gradient force,the low concentration salt water still moves as a heterogeneous flow and concentrates at the bottom.The transformation distance of the heterogeneous flow form of low concentration salt water is farther compared with that of high concentration salt water,and its kinematic state can be divided into the sliding stage and self-similarity stage effected by the bottom-bed drag force.The density Froude number is constant in the sliding stage (FrH=0.32),and decreases linearly with time in the self-similarity stage (slope of -0.01).