The invention discloses a method for harvesting microalgae through alkalescence flocculation-foam separation and a separation device. The method comprises the following steps: 1) adjusting the pH value of an algae liquid; 2) performing flocculating settling on the algae liquid in a settling pond after pH adjustment, thereby obtaining a flocculated algae liquid and supernate; and 3) introducing air into a foam separation tower which is filled with the flocculated algae liquid, a surfactant and a separation additive so as to enrich microalgae to foams, and collecting the foams, thereby obtaining the microalgae. The separation device comprises the settling pond and a circulation foam tower, wherein the flocculated algae liquid outlet in the settling pond is connected with an algae liquid inlet of the circulation foam tower through a middle storage tank. The circulation foam tower consists of a foam section and a rising section, and a circulation section is divided into a rising area and a settling area by using a guide cylinder, so that gas-liquid-solid contact mass transfer can be improved, and the separation efficiency can be improved. The method is high in recycling rate and concentration ratio, can operate continuously, is short in separation time and is applicable to large-scale industrial production, high in production capacity and relatively low in cost, and meanwhile circulation of water can be achieved.