Based on the model test, the aeration concentration of the water flow in a stilling pool with drop sill and sudden expansion was measured, and the influence of incoming flow conditions, sudden expansion ratio, and flow state on the aeration characteristics was studied. The results show that the typical flow pattern of the aerated water flow in a sudden-expansion slumping stilling pool is a hydraulic jump superimposed on the jet of which the head is split into two parts. The bubble zone is within the extension of the chute and the length is reduced compared with the stilling pool without sudden expansion. The length of the drip zone is increased, but its length near the bottom is reduced. A pure water recirculation zone containing bubbles occurs at the sudden expansion. The aeration concentration of the bottom plate of the stilling pool without sudden expansion shows a peak-decreasing curve distribution, and the aeration concentration of each longitudinal section is evenly distributed. The aeration concentration of the bottom plate of the stilling pool with sudden expansion shows a peak-rising curve distribution, with the highest aeration concentration at the 1/4 midline of the floor, the second highest concentration at the 1/2 midline of the floor, and the lowest concentration near the side walls. Under the same hydraulic conditions, the aeration concentration at the bottom of the sudden expansion stilling pool is significantly lower than that without sudden expansion, and the aeration concentration does not decrease with the increase of the sudden expansion ratio.