Abstract: The use of an annularaerator placing in the vertical shaft of flood discharge tunnel is a new engineering idea, and is different from the traditional ones.In order to better understand this new type of technology, the ventilation and aeration characteristics of the annular aerator are studied based on the prototype and modeling experiments carried out in the gyrating discharge tunnel of the Gongboxia Hydropower Project.The results show that the ventilation is caused by the pressure difference of air vents.The relationship between the ventilation and relative cavity length follows the linear distribution.The aeration effect on the annular aerator is obvious.The jet is striking water in the shaft, and then a large-scale aerated vortex is generated with bubbles thereby contributing to the aeration forming.The aeration concentration presents a power distribution in the vertical shaft.The ventilation and aeration scale effects of models are both obvious.Therefore, the use of an annular aerator is necessary to set in the vertical shaft of gyrating discharge tunnel while satisfying the structural designs demands, which can improve the flow pattern and increase the energy dissipation significantly.