The initiation movement of mud exposed to currents was investigated experimentally. The initial movement of mud with different sedimentary densities was determined by measuring the flow velocity and grey level of water’s image synchronously. A new method to determine the initiation movement of mud is provided (i.e., the method of relative grey value of water’s image in the flume). The experimental results show that the initiation movement of mud can be classified as slight erosion and chunk erosion. The sediment concentration on a muddy bed is low for the slight erosion, and the variation in water’s relative grey level is of the order of ∼10. The sediment concentration for the chunk erosion corresponding to a jump in the curve of relative grey value clearly increases, and the variation in relative grey value at that instant is larger than 20. The critical flow velocity, critical friction velocity, and critical shear stress of initiation of movement of mud increase with the increase in deposit density.