The invention relates to an enclosure type microalgae culture device and method. The device comprises an annular floating bed and an enclosure, wherein the enclosure is embedded in the floating bed; one side of the enclosure is connected with one side of the inner ring of the annular floating bed through a connecting shaft, so that the enclosure can rotate around the connecting shaft; and a connecting rope is tied to each of the other three sides of the enclosure, so that the enclosure can be tied to the corresponding sides of the inner ring of the annular floating bed. The method for culturing microalgae by using the device comprises the steps of: firstly, putting the device into a polluted water body, wherein one end of the enclosure, opposite to the connecting shaft, freely drops into water, and then the connecting ropes are tightened up, so that the polluted water body enters the enclosure; and then sterilizing the water body in the enclosure, killing insects, adding a culture medium to culture microalgae, and collecting the microalgae water body after the microalgae are cultured for 5 to 7 days and the concentration of chlorophyll a reaches 1,000mu g/L. By combining the benefits of simple microalgae culture, carbon dioxide absorption, water body purification and the like, the microalgae culture cost is remarkably reduced.