Stepped spillway is one of the idea forms to increase oxygen content in water body for hydraulic and environmental engineering. The effect factors on aeration rate in the water flow is closely related with the height, slope of step and water depth of the flow over stepped spillway. The relation between aeration rate and the effect factors mentioned above is developed. The aeration rates of 35 working conditions in the water flows over stepped spillways are simulated using numerical model. The simulated results are compared with experimental data by Chanson. It has shown that the simulated results are fairly coincident with experimental data. The study provides solid theoretical fundament on optimum design of spillways to increase oxygen content in water body.