The invention relates to the field of mass cultivation of microalgae and specifically relates to a horizontal submerged cover type carbon supplement device and carbon supplement method for cultivating microalgae in an open pond. The horizontal submerged cover type carbon supplement device for cultivating microalgae in an open pond, comprises: one or more than two covers and gas distributors, wherein the gas distributor is arranged at the bottom of the open pond and the cover is arranged above the gas distributor; the cover, in form of a cylinder with openings at both ends, is arranged along the flow direction of the culture solution, the two ends of the cover being a culture solution inlet and a culture solution outlet respectively; the gas distributor is kept away from the culture solution outlet of the cover by more than 40 cm. According to the invention, when microalgae cells are cultivated in the open pond, carbon dioxide can be effectively utilized for carbon supplement, so production cost is greatly reduced.