An axial flow impeller includes a hub, at least three base blades, and a cover blade detachably mounted to each base blade. When the impeller is applied in a medium environment having a high density and viscosity, the impeller will meet the requirement only by virtue of the base blade with the cover blade being removed. When the impeller is applied in a medium environment having low density and viscosity, the cover blade can be mounted in the front and/or rear of the base blade in order to improve the efficiency of the impeller. In this case, the consumption of electrical power can be reduced significantly while producing an equivalent propulsive force, thereby being advantageous for energy conservation. Therefore, the impeller can be adaptable to different medium environments and efficiency requirements, and thus can have good adaptability, need minimal investment cost and can be convenient to use.