The threat of dissolved gas supersaturation to fish has become an important ecological risk associating with operation of high dams. Enhancing flow turbulence and increasing mass transfer area are effective measures to promote the release of the supersaturated gas, which can be achieved by application of stepped spillways. This paper reports a preliminary study on the relationship between the mitigation of dissolved gas supersaturation in stepped spillway flows and their hydraulic and self-aeration properties. In this work, we focused on the transition flow regime on  a 45° sloping stepped spillway, with the ratio of critical depth to step height 1.13 < dc/h < 0.7. The gas saturation was measured along the stepped spillway together with the characteristics of flow aeration, including the distributions of void fraction, bubble count rate, mean bubble size, and specific air-water interfacial area. The results show that, within the range of transition flow conditions, the bubble count rate and air-water interfacial area increase with decreasing flow rate, and the enhancement of air-water mass transfer promotes the release of supersaturated gas from water flow. The experimental data is useful for quantitative analysis and theoretical modelling of correlation between the mass transfer area in turbulent multiphase flow and the supersaturated gas release efficiency in dissolved gas supersaturated water.