Abstract: Pre-aerator stepped spillways, where the aerator is placed prior to the primary step in the stepped spillway, can form a cavity chamber at the bottom to produce enough aeration concentration to avoid the cavitation erosion. To make a better understanding of the bottom aeration characteristics along the pre-aerator stepped spillways, laboratory experiments were carried out, in which aeration concentration distributions on the vertical and horizontal planes along the stepped spillway with different discharges and slopes were investigated. Ulteriorly, comparation of aeration characteristics was made between the pre-aerator stepped spillway and the transitional stepped spillway. It was showed that distribution of aeration concentration on the horizontal surfaces of the pre-aerator stepped spillway was similar to that on the vertical plane. Because of the pre-aerator, the aeration concentration is very high along the primary steps, decreases along the stepped spillways, and tends to the stability for a certain distance. With the increase of discharge per unit width and the decrease of the slope, the aeration concentration decreases. The results indicate that the pre-aerator can eliminate the clear water zone on the bottom of the primary steps, where the transitional steps cannot avoid. Therefore the pre-aerator stepped spillways can be widely used for larger discharges per unit width.