A combined mill wheel automatic aerator belongs to the life saving and fosterage equipment field in aquaculture. In the prior mill wheel automatic aerator, an overall stent is linked with a floating body with inconvenient dismounting and combination as well as the poor generality. The invention comprises a floating body, a power equipment, a reduction gearbox and an impeller. The power equipment is connected to the reduction gearbox, and the impeller is driven through an output shaft by the reduction gearbox, which is characterized in that the floating body is a closed hollow body comprising the vertical and horizontal floating bodies in vertical linkage. One end of the vertical floating body is linked on the horizontal floating body. The output shaft is supported by the floating body, and the impeller is disposed on the region enclosed by the vertical and horizontal bodies. In addition, the impeller of the automatic aerator is a stack mounting impeller formed from the series connection of a plurality of single-impellers. The beneficial effect of the invention is that the amounts of the floating body and the impeller can be changed based on the different needs of the actual oxygenaeration power. The flexible combination and installation are implemented with convenient dismantling, excellent generality, and a broader scope of applications. The plastic materials are adopted formanufacturing with light weight, good floating performance and low manufacturing costs.